Methods for manufacturing cold seal fluid-filled display apparatus

ABSTRACT

This methods and devices described herein relate to displays and methods of manufacturing cold seal fluid-filled displays, including MEMS. The fluid substantially surrounds the moving components of the MEMS display to reduce the effects of stiction and to improve the optical and electromechanical performance of the display. The invention relates to a method for sealing a MEMS display at a lower temperature such that a vapor bubble does not form forms only at temperatures about 15° C. to about 20° C. below the seal temperature. In some embodiments, the MEMS display apparatus includes a first substrate, a second substrate separated from the first substrate by a gap and supporting an array of light modulators, a fluid substantially filling the gap, a plurality of spacers within the gap, and a sealing material joining the first substrate to the second substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/221,606, filed Aug. 4, 2008, and claims the benefit of U.S.Provisional Patent Application Nos. 61/300,731 and 61/301,015, filedFeb. 2, 2010 and Feb. 3, 2010, respectively. The contents of each ofthese applications are incorporated by reference herein in theirentirety.

FIELD OF THE INVENTION

The invention generally relates to the field of displays, such asimaging and projection displays. In particular, the invention relates tothe assembly and operation of fluid filled display apparatus.

BACKGROUND OF THE INVENTION

Displays that incorporate mechanical light modulators can includehundreds, thousands, or in some cases, millions of moving elements. Insome devices, every movement of an element provides an opportunity forstatic friction to disable one or more of the elements. This movement isfacilitated by immersing all the parts in a working fluid (also referredto as fluid) and sealing the fluid (e.g., with an adhesive) within afluid space or gap in a MEMS display cell. The fluid is usually one witha low coefficient of friction, low viscosity, and minimal degradationeffects over the long term.

Because the fluid may possess a thermal expansion coefficient which isdifferent from that of the substrates which contain the fluid,variations in operating temperature can lead to strong variations in thefluid pressure within the display. These internal pressure variationscan lead to bulging or warping of the display surface and in some casesto the formation of vapor bubbles within the display. For example, theCTE of the glass that may be used for a MEMS substrate may be about 3.5ppm/K and the volumetric CTE for a suitable working fluid may be about1200 ppm/K. Additionally, the CTE of the adhesive used to seal the fluidin the display can govern the expansion of the cell gap. In someembodiments, the CTE of the adhesive may be about 80 ppm/K. Thus, theworking fluid in the gap in the MEMs display expands and contractsroughly 400 times more than that of the glass and 15 times more thanthat of the adhesive. For an approximately 50 degree Celsius temperaturedifference, the volume difference between the substrate and that of thefluid is about 5.5%. Thus, if the display is sealed at about 20° C. andis later heated to about 80° C., the fluid will expand about 5.5% morethan the glass MEMS substrate, which is in turn dominated by theadhesive swelling or expanding. The differences in expansion cause aforce to be exerted on the glass, resulting in a swelling of a portionof the display. This swelling amount is difficult to estimate accuratelyaround the edges of the MEMs display because the glass MEMS substrate isconfined around the edges by the adhesive and is generally free todeform in the center. In some embodiments, this swelling may in thecenter of the gap in the MEMS display may be as large as about 1.5microns. A similar effect may occur as the MEMS display is cooled. Ifthe display is cooled about 50° C., then the same fluid volumedifference of about 5.5% will result.

SUMMARY OF THE INVENTION

The methods and apparatus described herein allow for the manufacture ofimaging displays in which the formation of vapor bubbles issubstantially prevented.

When the display cell's internal pressure reduces below that of the sealpressure, and no prior vapor bubbles are present in the display, a vaporbubble forms. That is, one or more vapor bubbles nucleate suddenly inmultiple locations of the display. If the display is sealed at roomtemperature, a vapor bubble may form at temperatures far below roomtemperature. In practice, vapor bubbles do not appear until the displayreaches about 15° C. to 20° C. below the seal temperature. The actualtemperature at which a bubble forms is hard to predict as it depends onthe internal pressure and the ability of the spacers to absorb some ofthe contraction and/or expansion.

Conditions for vapor bubble formation generally exist when the cell isrestrained from contracting along with the reduced volume of fluid inthe gap of the display cell. When the ambient temperature of the displaycell is lowered, the substrate begins to contract. However, whenopposing spacers come into contact, i.e., the spacers on a firstsubstrate of the cell contact adjacent spacers on the second substrate(e.g., aperture plate) of the cell, the display cell becomes constrainedand cannot contract much further, while the fluid continues to contract.This constraint in turn reduces the pressure inside the display cell. Ifthe temperature continues to drop, then the pressure inside the cell maybe such that a vapor bubble forms. When vapor bubbles form within theoptical portion that the user looks at, they become an annoyance,usually leading to the replacement of the display.

The apparatuses and methods described herein relate to solutions that,inter alia, help prevent vapor bubble formation at low temperature. Inparticular, the methods and apparatuses described herein relate to theassembly of mechanically actuated display apparatus that include afluid-filling and a sealing process that occurs at lower temperatures. Amethod for sealing a MEMS display cell at a lower temperature, e.g.,below 0° C., and preferably between about −10° C. to about −25° C., isdescribed. This method takes into account the knowledge that a vaporbubble does not form immediately when the ambient temperature dropsbelow the seal temperature, but instead forms about 15° C. to about 20°C. below the seal temperature. In this manner, the temperature at whicha vapor bubble may form is lowered to lower than about −25° C.

Those skilled in the art will realize that standard sealing techniquesare performed at room temperature, while the sealing process describedherein is performed at a temperature substantially below roomtemperature (i.e., a cold temperature), and thus is a “cold seal”process for the manufacturing of a display apparatus is describedherein.

In one aspect, the invention relates to a method for manufacturing adisplay assembly including a first transparent substrate and a secondtransparent substrate. The method includes providing at least a portionof an array of light modulators on the second transparent substrate. Themethod further includes providing a plurality of spacers connected tothe first and second substrates to establish a gap between the twosubstrates. The method also includes providing an adhesive edge seal forbonding the perimeter of the first and second substrates, and fillingthe display assembly with a fluid at a first temperature. The methodfurther includes cooling the display assembly to a second temperaturesubstantially below the first temperature, and compressing the displayassembly thereby pushing the first and second substrates at leastpartially together. The method also includes curing a seal material toseal the fluid between the first and second substrates.

In some embodiments, the compressing step is performed at the secondtemperature. In some embodiments, the plurality of spacers maintains atleast a first gap between the two substrates. In some embodiments, theadhesive edge seal maintains the edges of the first and secondsubstrates separated by a second gap. In some embodiments, the adhesiveedge seal includes at least one edge spacer.

In some embodiments, the method further includes applying the sealmaterial to a fill hole located along an edge of the display assemblyafter the display assembly is filled and before the display assembly isreturned to room temperature. In some embodiments, the fill holecomprises an opening in the adhesive edge seal. In some embodiments,filling the display via the fill hole is performed such that the fluidsubstantially surrounds the movable portions of the light modulators atthe first temperature.

In some embodiments, the first temperature is substantially roomtemperature. In some embodiments, the first temperature is between about18° C. and about 30° C. In some embodiments, the second temperature isbelow about 0° C. In some embodiments, the curing the seal materialoccurs at a temperature below about 0° C. In some embodiments, thesecond temperature is between about −10° C. and −25° C.

In some embodiments, the fluid is one of a liquid, a gas, and alubricant. In some embodiments, the fluid comprises a hydrofluoroetherliquid. In some embodiments, the fluid comprises a liquid blend of atleast one perfluorocarbon and at least one hydrofluoroether.

In some embodiments, the light modulators are MEMS light modulators. Insome embodiments, the method further includes providing at least oneadditional array of MEMS light modulators on the first transparentsubstrate.

In some embodiments, the method further includes fabricating a pluralityof spacers on at least one of the first and second transparentsubstrates to maintain a gap between the two substrates. In someembodiments, the plurality of spacers maintain at least a first gapbetween the two substrates, wherein the adhesive edge seal maintains theedges of the first and second substrates separated by a second gap, andwherein the height of the second gap is greater than the height of thefirst gap. In some embodiments, the height of the second gap is largerthan the height of the first gap by between about 0.5 microns and about4 microns. In some embodiments, the height of the second gap is betweenabout 8 microns and about 14 microns.

In another aspect, the invention relates to a method for manufacturing adisplay assembly including a first transparent substrate and a secondtransparent substrate. The method includes providing at least a portionof an array of light modulators on the second transparent substrate. Themethod further includes providing a plurality of spacers connected tothe first and second substrates to establish a gap between the twosubstrates. The method also includes providing an adhesive edge seal forbonding the perimeter of the first and second substrates. The methodfurther includes compressing the display assembly thereby pushing thefirst and second substrates at least partially together, wherein thecompressing occurs at a temperature substantially below roomtemperature. The method also includes curing a seal material to seal afluid between the first and second substrates.

In some embodiments, room temperature is between about 18° C. and about30° C. In some embodiments, substantially below room temperature isbelow about 0° C. In some embodiments, substantially below roomtemperature is between about −10° C. and about −25° C. In someembodiments, the curing of the seal material at least partially occursat a temperature substantially below room temperature.

In some embodiments, the plurality of spacers maintain at least a firstgap between the two substrates. In some embodiments, the adhesive edgeseal maintains the edges of the first and second substrates separated bya second gap. In some embodiments, the adhesive edge seal includes atleast one edge spacer.

In some embodiments, the method further includes applying the sealmaterial to a fill hole located along an edge of the display assemblybefore the display assembly is returned to room temperature. In someembodiments, the fill hole comprises an opening in the adhesive edgeseal.

In a third aspect, the invention relates to a display apparatus. Thedisplay apparatus includes a first substrate, and a second substrateincluding at least a portion of an array of light modulators that isseparated from the first substrate by at least a first gap. The displayapparatus also includes a plurality of spacers connected to the firstand second substrates to maintain the first gap, and an adhesive edgeseal to maintain the edges of the display apparatus separated by atleast a second gap. The height of the second gap is greater than theheight of the first gap. The display apparatus further includes a fluidcontained within the first gap and a cured seal material to seal thefluid in the first gap. In some embodiments, the apparatus furtherincludes a fill hole, and the fill-hole comprises an opening in theadhesive edge seal.

In some embodiments, the fluid is one of a liquid, a gas, and alubricant. In some embodiments, the fluid comprises a hydrofluoroetherliquid. In some embodiments, the fluid comprises a liquid blend of atleast one perfluorocarbon and at least one hydrofluoroether.

In some embodiments, the light modulators are MEMS light modulators. Insome embodiments, the MEMS light modulators comprise shutter-based lightmodulators. In some embodiments, the MEMS light modulators compriseelectrowetting light modulators. In some embodiments, the lightmodulators comprise liquid crystal modulators. In some embodiments, thefirst transparent substrate includes an additional portion of an arrayof light modulators.

In some embodiments, the plurality of spacers are fabricated on one ofthe first and second transparent substrates. In some embodiments, thefirst substrate comprises one of a color filter array or an aperturelayer formed thereon. In some embodiments, the upper and lowersubstrates are electrically isolated. In some embodiments, the height ofthe second gap is larger than the height of the first gap by betweenabout 0.5 microns and 4 microns. In some embodiments, the height of thesecond gap is between about 8 microns and 14 microns. In someembodiments, the adhesive edge seal is an epoxy seal. In someembodiments, the epoxy seal is curable using an ultraviolet lightsource. In some embodiments, the adhesive edge seal includes at leastone edge spacer.

The illustrative descriptions herein include methods that can be used toconstrain the contraction of the MEMS substrate such that bubbleformation at lower temperatures is further reduced. For instance, byutilizing spacers such that opposing spacers do not completely contactuntil very low temperatures are reached, vapor bubble formation withinthe display apparatus temperature may be further reduced. In someembodiments, the display cell may be sealed with an adhesive materialwith a height substantially larger than that of each of the spacersinside the display cell. The adhesive may be an epoxy material. The sealmay be located at an edge of the display cell. In some embodiments, theseal material may include spacers made of plastic, glass, ceramic orother material. The spacers may be incompressible. In some embodiments,the spacer may be any suitable microstructure. Suitable microstructuresinclude a bead or a sphere. The bead or sphere may be formed from glassor silica.

In some embodiments, the seal material maintains the minimum separationor the cell gap between the substrates in the region of the substratesnear the seal, even under compression. In some embodiments, themicrostructure included with the seal material maintains the minimumseparation or a cell gap between the substrates in the region of thesubstrates near the seal material, even under compression.

When the cell of the display apparatus is compressed (e.g., by a cellpress), the majority of the spacers come into contact with theirrespective opposing spacers. However, if the cell press pressure is nottoo high and the seal material is larger than the total height of theopposing spacers on the substrates, then some of the spacers will notcontact the substrate along the extreme edges of the display cell,possibly due to the location of the seal material incompressible spacersat the extreme edges of the display cell. This in turn allows forfurther cell volume reduction as the fluid volume decreases.Nevertheless, as the temperature drops even further, eventually all oralmost all of the spacers will come into contact with the substrateand/or the cell will contract to its minimum allowable state.

In some embodiments, the displays are assembled using manufacturingequipment that includes a cassette that will perform the cooling of thedisplay cells. In some embodiments, the manufacturing equipment hasbuilt-in cell presses for compressing each display cell. In someembodiments, the display cell gap is filled with fluid at roomtemperature and only the pressing and sealing of the display is carriedout at cold temperatures.

In this application, implementations will primarily be described withrespect to displays built from MEMS light modulators. However, thesystems, methods, and methods and devices disclosed herein areapplicable to other types of displays, including electrowetting andliquid crystal displays, and more generally to apparatuses which includea fluid disposed therein. Examples of alternate MEMS-based lightmodulators include digital mirror devices (DMDs), interferencemodulation displays (IMODs), and light tap displays or frustratedinternal reflection displays.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing discussion will be understood more readily from thefollowing detailed description of the invention with reference to thefollowing drawings

FIG. 1A is an isometric view of display apparatus, according to anillustrative embodiment of the invention;

FIG. 1B is a block diagram of the display apparatus of FIG. 1A,according to an illustrative embodiment of the invention;

FIG. 2A is a perspective view of an illustrative shutter-based lightmodulator suitable for incorporation into the MEMS-based display of FIG.1A, according to an illustrative embodiment of the invention;

FIG. 2B is a cross-sectional view of a rollershade-based light modulatorsuitable for incorporation into the MEMS-based display of FIG. 1A,according to an illustrative embodiment of the invention;

FIG. 2C is a cross sectional view of a light-tap-based light modulatorsuitable for incorporation into an alternative embodiment of theMEMS-based display of FIG. 1A, according to an illustrative embodimentof the invention;

FIG. 2D is a cross sectional view of an electrowetting-based lightmodulator suitable for incorporation into an alternative embodiment ofthe MEMS-based display of FIG. 1A, according to an illustrativeembodiment of the invention;

FIG. 3A is a schematic diagram of a control matrix suitable forcontrolling the light modulators incorporated into the MEMS-baseddisplay of FIG. 1A, according to an illustrative embodiment of theinvention;

FIG. 3B is a perspective view of an array of shutter-based lightmodulators connected to the control matrix of FIG. 3A, according to anillustrative embodiment of the invention;

FIGS. 4A and 4B are plan views of a dual-actuated shutter assembly inthe open and closed states respectively, according to an illustrativeembodiment of the invention;

FIG. 5 is a cross-sectional view of a shutter-based display apparatus,according to an illustrative embodiment of the invention;

FIGS. 6A and 6B illustrate the structure of an aperture plate for use ina MEMS-down configuration, according to an illustrative embodiment ofthe invention;

FIG. 7 is a cross sectional view of a display, according to anillustrative embodiment of the invention.

FIG. 8 is a conceptual view of a precision substrate alignmentapparatus, according to an illustrative embodiment of the invention;

FIG. 9 is a plan view of a modulator substrate and an aperture platecomprising multiple modulator and aperture arrays respectively,according to an illustrative embodiment of the invention;

FIG. 10 is a plan view of a panel assembly after alignment, according toan illustrative embodiment of the invention;

FIG. 11 is a flow chart of a fluid-filled cell assembly method,according to an illustrative embodiment of the invention;

FIG. 12 is a view of a fluid filling apparatus, according toillustrative embodiments of the invention;

FIG. 13 is a flow chart of a fluid-filled cell assembly method formultiple arrays, according to an illustrative embodiment of theinvention;

FIG. 14 illustrates a cold seal method for assembling a displayapparatus, according to an illustrative embodiment of the invention;

FIG. 15 shows a MEMS display cell, with the aperture plate and the firstsubstrate substantially parallel, according to an illustrativeembodiment of the invention;

FIGS. 16-19 show display cells under compression, according toillustrative embodiments of the invention;

FIG. 17 shows a display cell in a condition after it has been furthercompressed at temperatures below the sealing temperature, according toan illustrative embodiment of the invention;

FIG. 18 shows a display cell in which the cell is under furthercompression from the cell pressure or from a colder temperature,according to an illustrative embodiment of the invention;

FIG. 19A shows a display assembly, according to an illustrativeembodiment of the invention, in which the spacers are made of an elasticmaterial are formed from materials which are chosen for a reducedmodulus of elasticity;

FIG. 19B shows a version of a display assembly under conditions in whichthe spacers maintain contact with one another even when the cell isallowed to relax at higher temperatures, according to an illustrativeembodiment of the invention;

FIG. 20 provides another illustration of display assembly in a conditionafter the assembly has been released from the press, sealed, and allowedto warm to room temperature, according to an illustrative embodiment ofthe invention.

DETAILED DESCRIPTION OF THE DRAWINGS

To provide an overall understanding of the invention, certainillustrative embodiments will now be described, including apparatus andmethods for displaying images. However, it will be understood by one ofordinary skill in the art that the systems and methods described hereinmay be adapted and modified as is appropriate for the application beingaddressed and that the systems and methods described herein may beemployed in other suitable applications, and that such other additionsand modifications will not depart from the scope hereof.

FIG. 1A is a schematic diagram of a direct-view MEMS-based displayapparatus 100, according to an illustrative embodiment of the invention.The display apparatus 100 includes a plurality of light modulators 102a-102 d (generally “light modulators 102”) arranged in rows and columns.In the display apparatus 100, light modulators 102 a and 102 d are inthe open state, allowing light to pass. Light modulators 102 b and 102 care in the closed state, obstructing the passage of light. Byselectively setting the states of the light modulators 102 a-102 d, thedisplay apparatus 100 can be utilized to form an image 104 for a backlitdisplay, if illuminated by a lamp or lamps 105. In anotherimplementation, the apparatus 100 may form an image by reflection ofambient light originating from the front of the apparatus. In anotherimplementation, the apparatus 100 may form an image by reflection oflight from a lamp or lamps positioned in the front of the display, i.e.by use of a frontlight. In one of the closed or open states, the lightmodulators 102 interfere with light in an optical path by, for example,and without limitation, blocking, reflecting, absorbing, filtering,polarizing, diffracting, or otherwise altering a property or path of thelight.

In the display apparatus 100, each light modulator 102 corresponds to apixel 106 in the image 104. In other implementations, the displayapparatus 100 may utilize a plurality of light modulators to form apixel 106 in the image 104. For example, the display apparatus 100 mayinclude three color-specific light modulators 102. By selectivelyopening one or more of the color-specific light modulators 102corresponding to a particular pixel 106, the display apparatus 100 cangenerate a color pixel 106 in the image 104. In another example, thedisplay apparatus 100 includes two or more light modulators 102 perpixel 106 to provide grayscale in an image 104. With respect to animage, a “pixel” corresponds to the smallest picture element defined bythe resolution of the image. With respect to structural components ofthe display apparatus 100, the term “pixel” refers to the combinedmechanical and electrical components utilized to modulate the light thatforms a single pixel of the image.

Display apparatus 100 is a direct-view display in that it does notrequire imaging optics. The user sees an image by looking directly atthe display apparatus 100. In alternate embodiments the displayapparatus 100 is incorporated into a projection display. In suchembodiments, the display forms an image by projecting light onto ascreen or onto a wall. In projection applications the display apparatus100 is substantially smaller than the projected image 104.

Direct-view displays may operate in either a transmissive or reflectivemode. In a transmissive display, the light modulators filter orselectively block light which originates from a lamp or lamps positionedbehind the display. The light from the lamps is optionally injected intoa light guide or “backlight”. Transmissive direct-view displayembodiments are often built onto transparent or glass substrates tofacilitate a sandwich assembly arrangement where one substrate,containing the light modulators, is positioned directly on top of thebacklight. In some transmissive display embodiments, a color-specificlight modulator is created by associating a color filter material witheach modulator 102. In other transmissive display embodiments colors canbe generated, as described below, using a field sequential color methodby alternating illumination of lamps with different primary colors.

Each light modulator 102 includes a shutter 108 and an aperture 109. Toilluminate a pixel 106 in the image 104, the shutter 108 is positionedsuch that it allows light to pass through the aperture 109 towards aviewer. To keep a pixel 106 unlit, the shutter 108 is positioned suchthat it obstructs the passage of light through the aperture 109. Theaperture 109 is defined by an opening patterned through a reflective orlight-absorbing material.

The display apparatus also includes a control matrix connected to thesubstrate and to the light modulators for controlling the movement ofthe shutters. The control matrix includes a series of electricalinterconnects (e.g., interconnects 110, 112, and 114), including atleast one write-enable interconnect 110 (also referred to as a“scan-line interconnect”) per row of pixels, one data interconnect 112for each column of pixels, and one common interconnect 114 providing acommon voltage to all pixels, or at least to pixels from both multiplecolumns and multiples rows in the display apparatus 100. In response tothe application of an appropriate voltage (the “write-enabling voltage,V_(we)”), the write-enable interconnect 110 for a given row of pixelsprepares the pixels in the row to accept new shutter movementinstructions. The data interconnects 112 communicate the new movementinstructions in the form of data voltage pulses. The data voltage pulsesapplied to the data interconnects 112, in some implementations, directlycontribute to an electrostatic movement of the shutters. In otherimplementations, the data voltage pulses control switches, e.g.,transistors or other non-linear circuit elements that control theapplication of separate actuation voltages, which are typically higherin magnitude than the data voltages, to the light modulators 102. Theapplication of these actuation voltages then results in theelectrostatic driven movement of the shutters 108.

FIG. 1B is a block diagram 150 of the display apparatus 100. Referringto FIGS. 1A and 1B, in addition to the elements of the display apparatus100 described above, as depicted in the block diagram 150, the displayapparatus 100 includes a plurality of scan drivers 152 (also referred toas “write enabling voltage sources”) and a plurality of data drivers 154(also referred to as “data voltage sources”). The scan drivers 152 applywrite enabling voltages to scan-line interconnects 110. The data drivers154 apply data voltages to the data interconnects 112. In someembodiments of the display apparatus, the data drivers 154 areconfigured to provide analog data voltages to the light modulators,especially where the gray scale of the image 104 is to be derived inanalog fashion. In analog operation the light modulators 102 aredesigned such that when a range of intermediate voltages is appliedthrough the data interconnects 112 there results a range of intermediateopen states in the shutters 108 and therefore a range of intermediateillumination states or gray scales in the image 104.

In other cases the data drivers 154 are configured to apply only areduced set of 2, 3, or 4 digital voltage levels to the control matrix.These voltage levels are designed to set, in digital fashion, either anopen state or a closed state to each of the shutters 108.

The scan drivers 152 and the data drivers 154 are connected to digitalcontroller circuit 156 (also referred to as the “controller 156”). Thecontroller 156 includes an input processing module 158, which processesan incoming image signal 157 into a digital image format appropriate tothe spatial addressing and the gray scale capabilities of the display100. The pixel location and gray scale data of each image is stored in aframe buffer 159 so that the data can be fed out as needed to the datadrivers 154. The data is sent to the data drivers 154 in mostly serialfashion, organized in predetermined sequences grouped by rows and byimage frames. The data drivers 154 can include series to parallel dataconverters, level shifting, and for some applications digital to analogvoltage converters.

The display 100 apparatus optionally includes a set of common drivers153, also referred to as common voltage sources. In some embodiments thecommon drivers 153 provide a DC common potential to all light modulatorswithin the array of light modulators 103, for instance by supplyingvoltage to a series of common interconnects 114. In other embodimentsthe common drivers 153, following commands from the controller 156,issue voltage pulses or signals to the array of light modulators 103,for instance global actuation pulses which are capable of driving and/orinitiating simultaneous actuation of all light modulators in multiplerows and columns of the array 103.

All of the drivers (e.g., scan drivers 152, data drivers 154, and commondrivers 153) for different display functions are time-synchronized by atiming-control module 160 in the controller 156. Timing commands fromthe module 160 coordinate the illumination of red, green and blue andwhite lamps (162, 164, 166, and 167 respectively) via lamp drivers 168,the write-enabling and sequencing of specific rows within the array ofpixels 103, the output of voltages from the data drivers 154, and theoutput of voltages that provide for light modulator actuation.

The controller 156 determines the sequencing or addressing scheme bywhich each of the shutters 108 in the array 103 can be re-set to theillumination levels appropriate to a new image 104. Details of suitableaddressing, image formation, and gray scale techniques can be found inU.S. patent application Ser. Nos. 11/326,696 and 11/643,042, theentireties of which are incorporated herein by reference. New images 104can be set at periodic intervals. For instance, for video displays, thecolor images 104 or frames of video are refreshed at frequencies rangingfrom 10 to 300 Hertz. In some embodiments the setting of an image frameto the array 103 is synchronized with the illumination of the lamps 162,164, and 166 such that alternate image frames are illuminated with analternating series of colors, such as red, green, and blue. The imageframes for each respective color is referred to as a color sub-frame. Inthis method, referred to as the field sequential color method, if thecolor sub-frames are alternated at frequencies in excess of 20 Hz, thehuman brain will average the alternating frame images into theperception of an image having a broad and continuous range of colors. Inalternate implementations, four or more lamps with primary colors can beemployed in display apparatus 100, employing primaries other than red,green, and blue.

In some implementations, where the display apparatus 100 is designed forthe digital switching of shutters 108 between open and closed states,the controller 156 determines the addressing sequence and/or the timeintervals between image frames to produce images 104 with appropriategray scale. The process of generating varying levels of grayscale bycontrolling the amount of time a shutter 108 is open in a particularframe is referred to as time division gray scale. In some embodiments oftime division gray scale, the controller 156 determines the time periodor the fraction of time within each frame that a shutter 108 is allowedto remain in the open state, according to the illumination level or grayscale desired of that pixel. In other implementations, for each imageframe, the controller 156 sets a plurality of sub-frame images inmultiple rows and columns of the array 103, and the controller altersthe duration over which each sub-frame image is illuminated inproportion to a gray scale value or significance value employed within acoded word for gray scale. For instance, the illumination times for aseries of sub-frame images can be varied in proportion to the binarycoding series 1, 2, 4, 8 . . . . The shutters 108 for each pixel in thearray 103 are then set to either the open or closed state within asub-frame image according to the value at a corresponding positionwithin the pixel's binary coded word for gray level.

In other implementations, the controller alters the intensity of lightfrom the lamps 162, 164, and 166 in proportion to the gray scale valuedesired for a particular sub-frame image. A number of hybrid techniquesare also available for forming colors and gray scale from an array ofshutters 108. For instance, the time division techniques described abovecan be combined with the use of multiple shutters 108 per pixel, or thegray scale value for a particular sub-frame image can be establishedthrough a combination of both sub-frame timing and lamp intensity.Details of these and other embodiments can be found in U.S. patentapplication Ser. No. 11/643,042, referenced above.

In some implementations the data for an image state 104 is loaded by thecontroller 156 to the modulator array 103 by a sequential addressing ofindividual rows, also referred to as scan lines. For each row or scanline in the sequence, the scan driver 152 applies a write-enable voltageto the write enable interconnect 110 for that row of the array 103, andsubsequently the data driver 154 supplies data voltages, correspondingto desired shutter states, for each column in the selected row. Thisprocess repeats until data has been loaded for all rows in the array. Insome implementations the sequence of selected rows for data loading islinear, proceeding from top to bottom in the array. In otherimplementations the sequence of selected rows is pseudo-randomized, inorder to minimize visual artifacts. And in other implementations thesequencing is organized by blocks, where, for a block, the data for onlya certain fraction of the image state 104 is loaded to the array, forinstance by addressing only every 5^(th) row of the array in sequence.

In some implementations, the process for loading image data to the array103 is separated in time from the process of actuating the shutters 108.In these implementations, the modulator array 103 may include datamemory elements for each pixel in the array 103 and the control matrixmay include a global actuation interconnect for carrying triggersignals, from common driver 153, to initiate simultaneous actuation ofshutters 108 according to data stored in the memory elements. Variousaddressing sequences, many of which are described in U.S. patentapplication Ser. No. 11/643,042, can be coordinated by means of thetiming control module 160.

In alternative embodiments, the array of pixels 103 and the controlmatrix that controls the pixels may be arranged in configurations otherthan rectangular rows and columns. For example, the pixels can bearranged in hexagonal arrays or curvilinear rows and columns. Ingeneral, as used herein, the term scan-line shall refer to any pluralityof pixels that share a write-enabling interconnect.

The display 100 is comprised of a plurality of functional blocksincluding the timing control module 160, the frame buffer 159, scandrivers 152, data drivers 154, and drivers 153 and 168. Each block canbe understood to represent either a distinguishable hardware circuitand/or a module of executable code. In some implementations thefunctional blocks are provided as distinct chips or circuits connectedtogether by means of circuit boards and/or cables. Alternately, many ofthese circuits can be fabricated along with the pixel array 103 on thesame substrate of glass or plastic. In other implementations, multiplecircuits, drivers, processors, and/or control functions from blockdiagram 150 may be integrated together within a single silicon chip,which is then bonded directly to the transparent substrate holding pixelarray 103.

The controller 156 includes a programming link 180 by which theaddressing, color, and/or gray scale algorithms, which are implementedwithin controller 156, can be altered according to the needs ofparticular applications. In some embodiments, the programming link 180conveys information from environmental sensors, such as ambient light ortemperature sensors, so that the controller 156 can adjust imaging modesor backlight power in correspondence with environmental conditions. Thecontroller 156 also comprises a power supply input 182 which providesthe power needed for lamps as well as light modulator actuation. Wherenecessary, the drivers 152, 153, 154, and/or 168 may include or beassociated with DC-DC converters for transforming an input voltage at182 into various voltages sufficient for the actuation of shutters 108or illumination of the lamps, such as lamps 162, 164, 166, and 167.

MEMS Light Modulators

FIG. 2A is a perspective view of an illustrative shutter-based lightmodulator 200 suitable for incorporation into the MEMS-based displayapparatus 100 of FIG. 1A, according to an illustrative embodiment of theinvention. The shutter-based light modulator 200 (also referred to asshutter assembly 200) includes a shutter 202 coupled to an actuator 204.The actuator 204 is formed from two separate compliant electrode beamactuators 205 (the “actuators 205”), as described in U.S. patentapplication Ser. No. 11/251,035, filed on Oct. 14, 2005. The shutter 202couples on one side to the actuators 205. The actuators 205 move theshutter 202 transversely over a surface 203 in a plane of motion whichis substantially parallel to the surface 203. The opposite side of theshutter 202 couples to a spring 207 which provides a restoring forceopposing the forces exerted by the actuator 204.

Each actuator 205 includes a compliant load beam 206 connecting theshutter 202 to a load anchor 208. The load anchors 208 along with thecompliant load beams 206 serve as mechanical supports, keeping theshutter 202 suspended proximate to the surface 203. The load anchors 208physically connect the compliant load beams 206 and the shutter 202 tothe surface 203 and electrically connect the load beams 206 to a biasvoltage, in some instances, ground.

Each actuator 205 also includes a compliant drive beam 216 positionedadjacent to each load beam 206. The drive beams 216 couple at one end toa drive beam anchor 218 shared between the drive beams 216. The otherend of each drive beam 216 is free to move. Each drive beam 216 iscurved such that it is closest to the load beam 206 near the free end ofthe drive beam 216 and the anchored end of the load beam 206.

The surface 203 includes one or more apertures 211 for admitting thepassage of light. If the shutter assembly 200 is formed on an opaquesubstrate, made for example from silicon, then the surface 203 is asurface of the substrate, and the apertures 211 are formed by etching anarray of holes through the substrate. If the shutter assembly 200 isformed on a transparent substrate, made for example of glass or plastic,then the surface 203 is a surface of a light blocking layer deposited onthe substrate, and the apertures are formed by etching the surface 203into an array of holes 211. The apertures 211 can be generally circular,elliptical, polygonal, serpentine, or irregular in shape.

In operation, a display apparatus incorporating the light modulator 200applies an electric potential to the drive beams 216 via the drive beamanchor 218. A second electric potential may be applied to the load beams206. The resulting potential difference between the drive beams 216 andthe load beams 206 pulls the free ends of the drive beams 216 towardsthe anchored ends of the load beams 206, and pulls the shutter ends ofthe load beams 206 toward the anchored ends of the drive beams 216,thereby driving the shutter 202 transversely towards the drive anchor218. The compliant members 206 act as springs, such that when thevoltage across the beams 206 and 216 is removed, the load beams 206 pushthe shutter 202 back into its initial position, releasing the stressstored in the load beams 206.

The shutter assembly 200, also referred to as an elastic shutterassembly, incorporates a passive restoring force, such as a spring, forreturning a shutter to its rest or relaxed position after voltages havebeen removed. A number of elastic restore mechanisms and variouselectrostatic couplings can be designed into or in conjunction withelectrostatic actuators, the compliant beams illustrated in shutterassembly 200 being just one example. Other examples are described inU.S. patent application Ser. Nos. 11/251,035 and 11/326,696, theentireties of which are incorporated herein by reference. For instance,a highly non-linear voltage-displacement response can be provided whichfavors an abrupt transition between “open” vs. “closed” states ofoperation, and which, in many cases, provides a bi-stable or hystericoperating characteristic for the shutter assembly. Other electrostaticactuators can be designed with more incremental voltage-displacementresponses and with considerably reduced hysteresis, as may be preferredfor analog gray scale operation.

The actuator 205 within the elastic shutter assembly is said to operatebetween a closed or actuated position and a relaxed position. Thedesigner, however, can choose to place apertures 211 such that shutterassembly 200 is in either the “open” state, i.e. passing light, or inthe “closed” state, i.e. blocking light, whenever actuator 205 is in itsrelaxed position. For illustrative purposes, it is assumed below thatelastic shutter assemblies described herein are designed to be open intheir relaxed state.

In many cases it is preferable to provide a dual set of “open” and“closed” actuators as part of a shutter assembly so that the controlelectronics are capable of electrostatically driving the shutters intoeach of the open and closed states.

Display apparatus 100, in alternative embodiments, includes lightmodulators other than transverse shutter-based light modulators, such asthe shutter assembly 200 described above. For example, FIG. 2B is across-sectional view of a rolling actuator shutter-based light modulator220 suitable for incorporation into an alternative embodiment of theMEMS-based display apparatus 100 of FIG. 1A, according to anillustrative embodiment of the invention. As described further in U.S.Pat. No. 5,233,459, entitled “Electric Display Device,” and U.S. Pat.No. 5,784,189, entitled “Spatial Light Modulator,” the entireties ofwhich are incorporated herein by reference, a rolling actuator-basedlight modulator includes a moveable electrode disposed opposite a fixedelectrode and biased to move in a preferred direction to produce ashutter upon application of an electric field. In some embodiments, thelight modulator 220 includes a planar electrode 226 disposed between asubstrate 228 and an insulating layer 224 and a moveable electrode 222having a fixed end 230 attached to the insulating layer 224. In theabsence of any applied voltage, a moveable end 232 of the moveableelectrode 222 is free to roll towards the fixed end 230 to produce arolled state. Application of a voltage between the electrodes 222 and226 causes the moveable electrode 222 to unroll and lie flat against theinsulating layer 224, whereby it acts as a shutter that blocks lighttraveling through the substrate 228. The moveable electrode 222 returnsto the rolled state by means of an elastic restoring force after thevoltage is removed. The bias towards a rolled state may be achieved bymanufacturing the moveable electrode 222 to include an anisotropicstress state.

FIG. 2C is a cross-sectional view of an illustrative non shutter-basedMEMS light modulator 250. The light tap modulator 250 is suitable forincorporation into an alternative embodiment of the MEMS-based displayapparatus 100 of FIG. 1A, according to an illustrative embodiment of theinvention. As described further in U.S. Pat. No. 5,771,321, entitled“Micromechanical Optical Switch and Flat Panel Display,” the entirety ofwhich is incorporated herein by reference, a light tap works accordingto a principle of frustrated total internal reflection. That is, light252 is introduced into a light guide 254, in which, withoutinterference, light 252 is for the most part unable to escape the lightguide 254 through its front or rear surfaces due to total internalreflection. The light tap 250 includes a tap element 256 that has asufficiently high index of refraction that, in response to the tapelement 256 contacting the light guide 254, light 252 impinging on thesurface of the light guide 254 adjacent the tap element 256 escapes thelight guide 254 through the tap element 256 towards a viewer, therebycontributing to the formation of an image.

In some embodiments, the tap element 256 is formed as part of beam 258of flexible, transparent material. Electrodes 260 coat portions of oneside of the beam 258. Opposing electrodes 260 are disposed on the lightguide 254. By applying a voltage across the electrodes 260, the positionof the tap element 256 relative to the light guide 254 can be controlledto selectively extract light 252 from the light guide 254.

FIG. 2D is a cross sectional view of a second illustrativenon-shutter-based MEMS light modulator suitable for inclusion in variousembodiments of the invention. Specifically, FIG. 2D is a cross sectionalview of an electrowetting-based light modulation array 270. Theelectrowetting-based light modulator array 270 is suitable forincorporation into an alternative embodiment of the MEMS-based displayapparatus 100 of FIG. 1A, according to an illustrative embodiment of theinvention. The light modulation array 270 includes a plurality ofelectrowetting-based light modulation cells 272 a-272 d (generally“cells 272”) formed on an optical cavity 274. The light modulation array270 also includes a set of color filters 276 corresponding to the cells272.

Each cell 272 includes a layer of water (or other transparent conductiveor polar fluid) 278, a layer of light absorbing oil 280, a transparentelectrode 282 (made, for example, from indium-tin oxide) and aninsulating layer 284 positioned between the layer of light absorbing oil280 and the transparent electrode 282. Illustrative implementations ofsuch cells are described further in U.S. Patent Application PublicationNo. 2005/0104804, published May 19, 2005 and entitled “Display Device,”the entirety of which is incorporated herein by reference In theembodiment described herein, the electrode takes up a portion of a rearsurface of a cell 272.

In order to increase switching speed, at least one of the two liquidcomponents 278 and 280 in the electrowetting display should have a lowviscosity, preferably less than 70 centipoise and more preferably lessthan 10 centipoise. Lower viscosities can be facilitated if at least oneof the two liquid components includes materials having molecular weightsless than 4000 grams/mole, preferably less than 400 grams/mole. Suitablelow viscosity fluids include water, alcohols, fluorinated silicone oils,polydimethylsiloxane, hexamethyldisiloxane, octamethyltrisiloxance,octane, and diethylbenzene.

Suitable low viscosity non-polar oils include, without limitation,paraffins, olefins, ethers, silicone oils, fluorinated silicone oils, orother natural or synthetic solvents or lubricants. Useful oils can bepolydimethylsiloxanes, such as hexamethyldisiloxane andoctamethyltrisiloxane, or alkyl methyl siloxanes such ashexylpentamethyldisiloxane. Useful oils can be alkanes, such as octaneor decane. Useful oils can be nitroalkanes, such as nitromethane. Usefuloils can be aromatic compounds, such as toluene or diethylbenzene.Useful oils can be ketones, such as butanone or methyl isobutyl ketone.Useful oils can be chlorocarbons, such as chlorobenzene. And useful oilscan be chlorofluorocarbons, such as dichlorofluoroethane orchlorotrifluoroethylene. The oils can be mixed with dyes to increaselight absorption, either at specific colors such as cyan, magenta, andyellow, or over a broader spectrum to create a black ink.

For many embodiments it is useful to incorporate mixtures of the aboveoils. For instance mixtures of alkanes or mixtures ofpolydimethylsiloxanes can be useful where the mixture includes moleculeswith a range of molecular weights. One can also optimize properties bymixing fluids from different families or fluids with differentproperties. For instance, the surface wetting properties of ahexamethyldisiloxane and be combined with the low viscosity of butanoneto create an improved fluid.

The light modulation array 270 also includes a light guide 288 and oneor more light sources 292 which inject light 294 into the light guide288. A series of light redirectors 291 are formed on the rear surface ofthe light guide, proximate a front facing reflective layer 290. Thelight redirectors 291 may be either diffuse or specular reflectors. Themodulation array 270 includes an aperture layer 286 which is patternedinto a series of apertures, one aperture for each of the cells 272, toallow light rays 294 to pass through the cells 272 and toward theviewer.

In some embodiments the aperture layer 286 is comprised of a lightabsorbing material to block the passage of light except through thepatterned apertures. In another embodiment the aperture layer 286 iscomprised of a reflective material which reflects light not passingthrough the surface apertures back towards the rear of the light guide288. After returning to the light guide, the reflected light can befurther recycled by the front facing reflective layer 290.

In operation, application of a voltage to the electrode 282 of a cellcauses the light absorbing oil 280 in the cell to move into or collectin one portion of the cell 272. As a result, the light absorbing oil 280no longer obstructs the passage of light through the aperture formed inthe reflective aperture layer 286 (see, for example, cells 272 b and 272c). Light escaping the light guide 288 at the aperture is then able toescape through the cell and through a corresponding color (for example,red, green, or blue) filter in the set of color filters 276 to form acolor pixel in an image. When the electrode 282 is grounded, the lightabsorbing oil 280 returns to its previous position (as in cell 272 a)and covers the aperture in the reflective aperture layer 286, absorbingany light 294 attempting to pass through it.

The roller-based light modulator 220, light tap 250, andelectrowetting-based light modulation array 270 are not the onlyexamples of MEMS light modulators suitable for inclusion in variousembodiments of the invention. It will be understood that other MEMSlight modulators can exist and can be usefully incorporated into theinvention.

U.S. patent application Ser. Nos. 11/251,035 and 11/326,696 havedescribed a variety of methods by which an array of shutters can becontrolled via a control matrix to produce images, in many cases movingimages, with appropriate gray scale. In some cases, control isaccomplished by means of a passive matrix array of row and columninterconnects connected to driver circuits on the periphery of thedisplay. In other cases it is appropriate to include switching and/ordata storage elements within each pixel of the array (the so-calledactive matrix) to improve either the speed, the gray scale and/or thepower dissipation performance of the display.

FIG. 3A is a schematic diagram of a control matrix 300 suitable forcontrolling the light modulators incorporated into the MEMS-baseddisplay apparatus 100 of FIG. 1A, according to an illustrativeembodiment of the invention. FIG. 3B is a perspective view of an array320 of shutter-based light modulators connected to the control matrix300 of FIG. 3A, according to an illustrative embodiment of theinvention. The control matrix 300 may address an array of pixels 320(the “array 320”). Each pixel 301 includes an elastic shutter assembly302, such as the shutter assembly 200 of FIG. 2A, controlled by anactuator 303. Each pixel also includes an aperture layer 322 thatincludes apertures 324. Further electrical and mechanical descriptionsof shutter assemblies such as shutter assembly 302, and variationsthereon, can be found in U.S. patent application Ser. Nos. 11/251,035and 11/326,696. Descriptions of alternate control matrices can also befound in U.S. patent application Ser. No. 11/607,715, the entirety ofwhich is incorporated herein by reference.

The control matrix 300 is fabricated as a diffused orthin-film-deposited electrical circuit on the surface of a substrate 304on which the shutter assemblies 302 are formed. The control matrix 300includes a scan-line interconnect 306 for each row of pixels 301 in thecontrol matrix 300 and a data-interconnect 308 for each column of pixels301 in the control matrix 300. Each scan-line interconnect 306electrically connects a write-enabling voltage source 307 to the pixels301 in a corresponding row of pixels 301. Each data interconnect 308electrically connects a data voltage source, (“Vd source”) 309 to thepixels 301 in a corresponding column of pixels 301. In control matrix300, the data voltage V_(d) provides the majority of the energynecessary for actuation of the shutter assemblies 302. Thus, the datavoltage source 309 also serves as an actuation voltage source.

Referring to FIGS. 3A and 3B, for each pixel 301 or for each shutterassembly 302 in the array of pixels 320, the control matrix 300 includesa transistor 310 and a capacitor 312. The gate of each transistor 310 iselectrically connected to the scan-line interconnect 306 of the row inthe array 320 in which the pixel 301 is located. The source of eachtransistor 310 is electrically connected to its corresponding datainterconnect 308. The actuators 303 of each shutter assembly 302 includetwo electrodes. The drain of each transistor 310 is electricallyconnected in parallel to one electrode of the corresponding capacitor312 and to one of the electrodes of the corresponding actuator 303. Theother electrode of the capacitor 312 and the other electrode of theactuator 303 in shutter assembly 302 are connected to a common or groundpotential. In alternate implementations, the transistors 310 can bereplaced with semiconductor diodes and or metal-insulator-metal sandwichtype switching elements.

In operation, to form an image, the control matrix 300 write-enableseach row in the array 320 in a sequence by applying V_(we) to eachscan-line interconnect 306 in turn. For a write-enabled row, theapplication of V_(we) to the gates of the transistors 310 of the pixels301 in the row allows the flow of current through the data interconnects308 through the transistors 310 to apply a potential to the actuator 303of the shutter assembly 302. While the row is write-enabled, datavoltages V_(d) are selectively applied to the data interconnects 308. Inimplementations providing analog gray scale, the data voltage applied toeach data interconnect 308 is varied in relation to the desiredbrightness of the pixel 301 located at the intersection of thewrite-enabled scan-line interconnect 306 and the data interconnect 308.In implementations providing digital control schemes, the data voltageis selected to be either a relatively low magnitude voltage (i.e., avoltage near ground) or to meet or exceed V_(at) (the actuationthreshold voltage). In response to the application of V_(at) to a datainterconnect 308, the actuator 303 in the corresponding shutter assembly302 actuates, opening the shutter in that shutter assembly 302. Thevoltage applied to the data interconnect 308 remains stored in thecapacitor 312 of the pixel 301 even after the control matrix 300 ceasesto apply V_(we) to a row. It is not necessary, therefore, to wait andhold the voltage V_(we) on a row for times long enough for the shutterassembly 302 to actuate; such actuation can proceed after thewrite-enabling voltage has been removed from the row. The capacitors 312also function as memory elements within the array 320, storing actuationinstructions for periods as long as is necessary for the illumination ofan image frame.

The pixels 301 as well as the control matrix 300 of the array 320 areformed on a substrate 304. The array includes an aperture layer 322,disposed on the substrate 304, which includes a set of apertures 324 forrespective pixels 301 in the array 320. The apertures 324 are alignedwith the shutter assemblies 302 in each pixel. In one implementation thesubstrate 304 is made of a transparent material, such as glass orplastic. In another implementation the substrate 304 is made of anopaque material, but in which holes are etched to form the apertures324.

Components of shutter assemblies 302 are processed either at the sametime as the control matrix 300 or in subsequent processing steps on thesame substrate. The electrical components in control matrix 300 arefabricated using many thin film techniques in common with themanufacture of thin film transistor arrays for liquid crystal displays.Available techniques are described in Den Boer, Active Matrix LiquidCrystal Displays (Elsevier, Amsterdam, 2005), the entirety of which isincorporated herein by reference. The shutter assemblies are fabricatedusing techniques similar to the art of micromachining or from themanufacture of micromechanical (i.e., MEMS) devices. Many applicablethin film MEMS techniques are described in Rai-Choudhury, ed., Handbookof Microlithography, Micromachining & Microfabrication (SPIE OpticalEngineering Press, Bellingham, Wash. 1997), the entirety of which isincorporated herein by reference. Fabrication techniques specific toMEMS light modulators formed on glass substrates can be found in U.S.patent application Ser. Nos. 11/361,785 and 11/731,628, the entiretiesof which are incorporated herein by reference. For instance, asdescribed in those applications, the shutter assembly 302 can be formedfrom thin films of amorphous silicon, deposited by a chemical vapordeposition process.

The shutter assembly 302 together with the actuator 303 can be madebi-stable. That is, the shutters can exist in at least two equilibriumpositions (e.g. open or closed) with little or no power required to holdthem in either position. More particularly, the shutter assembly 302 canbe mechanically bi-stable. Once the shutter of the shutter assembly 302is set in position, no electrical energy or holding voltage is requiredto maintain that position. The mechanical stresses on the physicalelements of the shutter assembly 302 can hold the shutter in place.

The shutter assembly 302 together with the actuator 303 can also be madeelectrically bi-stable. In an electrically bi-stable shutter assembly,there exists a range of voltages below the actuation voltage of theshutter assembly, which if applied to a closed actuator (with theshutter being either open or closed), holds the actuator closed and theshutter in position, even if an opposing force is exerted on theshutter. The opposing force may be exerted by a spring such as spring207 in shutter-based light modulator 200, or the opposing force may beexerted by an opposing actuator, such as an “open” or “closed” actuator.

The light modulator array 320 is depicted as having a single MEMS lightmodulator per pixel. Other embodiments are possible in which multipleMEMS light modulators are provided in each pixel, thereby providing thepossibility of more than just binary “on’ or “off” optical states ineach pixel. Certain forms of coded area division gray scale are possiblewhere multiple MEMS light modulators in the pixel are provided, andwhere apertures 324, which are associated with each of the lightmodulators, have unequal areas.

In other embodiments the roller-based light modulator 220, the light tap250, or the electrowetting-based light modulation array 270, as well asother MEMS-based light modulators, can be substituted for the shutterassembly 302 within the light modulator array 320.

FIGS. 4A and 4B illustrate an alternative shutter-based light modulator(shutter assembly) 400 suitable for inclusion in various embodiments ofthe invention. The light modulator 400 is an example of a dual actuatorshutter assembly, and is shown in FIG. 4A in an open state. FIG. 4B is aview of the dual actuator shutter assembly 400 in a closed state.Shutter assembly 400 is described in further detail in U.S. patentapplication Ser. No. 11/251,035, referenced above. In contrast to theshutter assembly 200, shutter assembly 400 includes actuators 402 and404 on either side of a shutter 406. Each actuator 402 and 404 isindependently controlled. A first actuator, a shutter-open actuator 402,serves to open the shutter 406. A second opposing actuator, theshutter-close actuator 404, serves to close the shutter 406. Bothactuators 402 and 404 are compliant beam electrode actuators. Theactuators 402 and 404 open and close the shutter 406 by driving theshutter 406 substantially in a plane parallel to an aperture layer 407over which the shutter is suspended. The shutter 406 is suspended ashort distance over the aperture layer 407 by anchors 408 attached tothe actuators 402 and 404. The inclusion of supports attached to bothends of the shutter 406 along its axis of movement reduces out of planemotion of the shutter 406 and confines the motion substantially to aplane parallel to the substrate. By analogy to the control matrix 300 ofFIG. 3A, a control matrix suitable for use with shutter assembly 400might include one transistor and one capacitor for each of the opposingshutter-open and shutter-close actuators 402 and 404.

The shutter 406 includes two shutter apertures 412 through which lightcan pass. The aperture layer 407 includes a set of three apertures 409.In FIG. 4A, the shutter assembly 400 is in the open state and, as such,the shutter-open actuator 402 has been actuated, the shutter-closeactuator 404 is in its relaxed position, and the centerlines ofapertures 412 and 409 coincide. In FIG. 4B the shutter assembly 400 hasbeen moved to the closed state and, as such, the shutter-open actuator402 is in its relaxed position, the shutter-close actuator 404 has beenactuated, and the light blocking portions of shutter 406 are now inposition to block transmission of light through the apertures 409 (shownas dotted lines).

Each aperture has at least one edge around its periphery. For example,the rectangular apertures 409 have four edges. In alternativeimplementations in which circular, elliptical, oval, or other curvedapertures are formed in the aperture layer 407, each aperture may haveonly a single edge. In other implementations the apertures need not beseparated or disjoint in the mathematical sense, but instead can beconnected. That is to say, while portions or shaped sections of theaperture may maintain a correspondence to each shutter, several of thesesections may be connected such that a single continuous perimeter of theaperture is shared by multiple shutters.

In order to allow light with a variety of exit angles to pass throughapertures 412 and 409 in the open state, it is advantageous to provide awidth or size for shutter apertures 412 which is larger than acorresponding width or size of apertures 409 in the aperture layer 407.In order to effectively block light from escaping in the closed state,it is preferable that the light blocking portions of the shutter 406overlap the apertures 409. FIG. 4B shows a predefined overlap 416between the edge of light blocking portions in the shutter 406 and oneedge of the aperture 409 formed in aperture layer 407.

The electrostatic actuators 402 and 404 are designed so that theirvoltage-displacement behavior provides a bi-stable characteristic to theshutter assembly 400. For each of the shutter-open and shutter-closeactuators there exists a range of voltages below the actuation voltage,which if applied while that actuator is in the closed state (with theshutter being either open or closed), will hold the actuator closed andthe shutter in position, even after an actuation voltage is applied tothe opposing actuator. The minimum voltage needed to maintain ashutter's position against such an opposing force is referred to as amaintenance voltage V_(m). A number of control matrices which takeadvantage of the bi-stable operation characteristic are described inU.S. patent application Ser. No. 11/607,715, referenced above.

FIG. 5 is a cross sectional view of a display apparatus 500incorporating shutter-based light modulators (shutter assemblies) 502,according to an illustrative embodiment of the invention. Each shutterassembly incorporates a shutter 503 and an anchor 505. Not shown are thecompliant beam actuators which, when connected between the anchors 505and the shutters 503, help to suspend the shutters a short distanceabove the surface. The shutter assemblies 502 are disposed on atransparent substrate 504, preferably made of plastic or glass. Arear-facing reflective layer, reflective film 506, disposed on thesubstrate 504 defines a plurality of surface apertures 508 locatedbeneath the closed positions of the shutters 503 of the shutterassemblies 502. The reflective film 506 reflects light not passingthrough the surface apertures 508 back towards the rear of the displayapparatus 500. The reflective aperture layer 506 can be a fine-grainedmetal film without inclusions formed in thin film fashion by a number ofvapor deposition techniques including sputtering, evaporation, ionplating, laser ablation, or chemical vapor deposition. In anotherimplementation, the rear-facing reflective layer 506 can be formed froma mirror, such as a dielectric mirror. A dielectric mirror is fabricatedas a stack of dielectric thin films which alternate between materials ofhigh and low refractive index. The vertical gap which separates theshutters 503 from the reflective film 506, within which the shutter isfree to move, is in the range of 0.5 to 10 microns. The magnitude of thevertical gap is preferably less than the lateral overlap between theedge of shutters 503 and the edge of apertures 508 in the closed state,such as the overlap 416 shown in FIG. 4B.

The display apparatus 500 includes an optional diffuser 512 and/or anoptional brightness enhancing film 514 which separate the substrate 504from a planar light guide 516. The light guide is comprised of atransparent, i.e. glass or plastic material. The light guide 516 isilluminated by one or more light sources 518, forming a backlight. Thelight sources 518 can be, for example, and without limitation,incandescent lamps, fluorescent lamps, lasers, or light emitting diodes(LEDs). A reflector 519 helps direct light from lamp 518 towards thelight guide 516. A front-facing reflective film 520 is disposed behindthe backlight 516, reflecting light towards the shutter assemblies 502.Light rays such as ray 521 from the backlight that do not pass throughone of the shutter assemblies 502 will be returned to the backlight andreflected again from the film 520. In this fashion light that fails toleave the display to form an image on the first pass can be recycled andmade available for transmission through other open apertures in thearray of shutter assemblies 502. Such light recycling has been shown toincrease the illumination efficiency of the display.

The light guide 516 includes a set of geometric light redirectors orprisms 517 which re-direct light from the lamps 518 towards theapertures 508 and hence toward the front of the display. The lightre-directors can be molded into the plastic body of light guide 516 withshapes that can be alternately triangular, trapezoidal, or curved incross section. The density of the prisms 517 generally increases withdistance from the lamp 518.

In alternate embodiments the aperture layer 506 can be made of a lightabsorbing material, and in alternate embodiments the surfaces of shutter503 can be coated with either a light absorbing or a light reflectingmaterial. In alternate embodiments the aperture layer 506 can bedeposited directly on the surface of the light guide 516. In alternateembodiments the aperture layer 506 need not be disposed on the samesubstrate as the shutters 503 and anchors 505 (see the MEMS-downconfiguration described below). These and other embodiments for adisplay illumination system are described in detail in the U.S. patentapplication Ser. Nos. 11/218,690 and 11/528,191, the entireties of whichare incorporated herein by reference.

In one implementation the light sources 518 can include lamps ofdifferent colors, for instance, the colors red, green, and blue. A colorimage can be formed by sequentially illuminating images with lamps ofdifferent colors at a rate sufficient for the human brain to average thedifferent colored images into a single multi-color image. The variouscolor-specific images are formed using the array of shutter assemblies502. In another implementation, the light source 518 includes lampshaving more than three different colors. For example, the light source518 may have red, green, blue and white lamps or red, green, blue, andyellow lamps.

A cover plate 522 forms the front of the display apparatus 500. The rearside of the cover plate 522 can be covered with a black matrix 524 toincrease contrast. In alternate implementations the cover plate includescolor filters, for instance distinct red, green, and blue filterscorresponding to different ones of the shutter assemblies 502. The coverplate 522 is supported a predetermined distance away from the shutterassemblies 502 forming a gap 526. The gap 526 is maintained bymechanical supports or spacers 527 and/or by an adhesive seal 528attaching the cover plate 522 to the substrate 504.

The adhesive seal 528 seals in a working fluid 530. The working fluid530 is engineered with viscosities preferably below about 10 centipoiseand with relative dielectric constant preferably above about 2.0, anddielectric breakdown strengths above about 10⁴ V/cm. The working fluid530 can also serve as a lubricant. In one implementation, the workingfluid 530 is a hydrophobic liquid with a high surface wettingcapability. In alternate implementations the working fluid 530 has arefractive index that is either greater than or less than that of thesubstrate 504.

When the MEMS-based display assembly includes a liquid for the workingfluid 530, the liquid at least partially surrounds the moving parts ofthe MEMS-based light modulator. In order to reduce the actuationvoltages, the liquid has a viscosity preferably below 70 centipoise,more preferably below 10 centipoise. Liquids with viscosities below 70centipoise can include materials with low molecular weights: below 4000grams/mole, or in some cases below 400 grams/mole. Suitable workingfluids 530 include, without limitation, de-ionized water, methanol,ethanol and other alcohols, paraffins, olefins, ethers, silicone oils,fluorinated silicone oils, or other natural or synthetic solvents orlubricants. Useful working fluids can be polydimethylsiloxanes, such ashexamethyldisiloxane and octamethyltrisiloxane, or alkyl methylsiloxanes such as hexylpentamethyldisiloxane. Useful working fluids canbe alkanes, such as octane or decane. Useful fluids can be nitroalkanes,such as nitromethane. Useful fluids can be aromatic compounds, such astoluene or diethylbenzene. Useful fluids can be ketones, such asbutanone or methyl isobutyl ketone. Useful fluids can be chlorocarbons,such as chlorobenzene. Useful fluids can be chlorofluorocarbons, such asdichlorofluoroethane or chlorotrifluoroethylene. And other fluidsconsidered for these display assemblies include butyl acetate,dimethylformamide.

For many embodiments it is advantageous to incorporate a mixture of theabove fluids. For instance mixtures of alkanes or mixtures ofpolydimethylsiloxanes can be useful where the mixture includes moleculeswith a range of molecular weights. It is also possible to optimizeproperties by mixing fluids from different families or fluids withdifferent properties. For instance, the surface wetting properties of ahexamethyldisiloxane and be combined with the low viscosity of butanoneto create an improved fluid.

A sheet metal or molded plastic assembly bracket 532 holds the coverplate 522, the substrate 504, the backlight 516 and the other componentparts together around the edges. The assembly bracket 532 is fastenedwith screws or indent tabs to add rigidity to the combined displayapparatus 500. In some implementations, the light source 518 is moldedin place by an epoxy potting compound. Reflectors 536 help return lightescaping from the edges of light guide 516 back into the light guide.Not shown in FIG. 5 are electrical interconnects which provide controlsignals as well as power to the shutter assemblies 502 and the lamps518.

Further details and alternate configurations for the display apparatus500, including manufacturing methods therefore, can be found in the U.S.patent application Ser. Nos. 11/361,785 and 11/731,628, the entiretiesof which are incorporated herein by reference.

In other embodiments, the roller-based light modulator 220, the lighttap 250, or the electrowetting-based light modulation array 270, as wellas other MEMS-based light modulators, can be substituted for the shutterassemblies 502 within the display assembly 500.

Display apparatus 500 is referred to as the MEMS-up configuration,wherein the MEMS based light modulators are formed on a front surface ofsubstrate 504, i.e. the surface that faces toward the viewer. Theshutter assemblies 502 are built directly on top of the reflectiveaperture layer 506. In an alternate embodiment of the invention,referred to as the MEMS-down configuration, the shutter assemblies aredisposed on a substrate separate from the substrate on which thereflective aperture layer is formed. The substrate on which thereflective aperture layer is formed, defining a plurality of apertures,is referred to herein as the aperture plate. In the MEMS-downconfiguration, the substrate that carries the MEMS-based lightmodulators takes the place of the cover plate 522 in display apparatus500 and is oriented such that the MEMS-based light modulators arepositioned on the rear surface of the top substrate, i.e. the surfacethat faces away from the viewer and toward the back light 516. TheMEMS-based light modulators are thereby positioned directly opposite toand across a gap from the reflective aperture layer. The gap can bemaintained by a series of spacer posts connecting the aperture plate andthe substrate on which the MEMS modulators are formed. In someimplementations the spacers are disposed within or between each pixel inthe array. The gap or distance that separates the MEMS light modulatorsfrom their corresponding apertures is preferably less than 10 microns,or a distance that is less than the overlap between shutters andapertures, such as overlap 416. Further details and alternateembodiments for the MEMS-down display configuration can be found in theU.S. patent application Ser. Nos. 11/361,785, 11/528,191, and 11/731,628referenced above.

The aperture plate 2700 of FIG. 6 illustrates the detailed structureswithin one implementation of an aperture plate, for use in a MEMS-downconfiguration. The aperture plate 2700 includes a substrate 2702, adielectrically enhanced metal mirror 2704, a light absorbing layer 2706,and a spacer post 2708. The dielectrically enhanced metal mirror and thelight absorbing layer have been patterned into apertures 2709.

The substrate 2702 is preferably a transparent material, for exampleglass or plastic. The dielectrically enhanced metal mirror 2704 iscomprised of a 5-layer stack of materials including, in order from thesubstrate up, a thin film of Si3N4 2710, a thin film of SiO2 2712,another thin film of Si3N4 2710, another thin film of SiO2, 2712, and athin film of aluminum 2714. The relative thicknesses and preferredrefractive indices of these layers are given in Table 1.

TABLE 1 Film Thicknesses and Refractive Indices for a DielectricallyEnhanced Metal Mirror. Thin film material Thickness Refractive index 5.Aluminum 200 nm or less NA 4. SiO2  88 nm 1.46 3. Si3N4  64 nm 2.0 2.SiO2  88 nm 1.46 1. Si3N4  64 nm 2.0

The light absorbing layer 2706 can be formed from a thin film of blackchrome, which is a composite of chromium metal particles suspended in anoxide or nitride matrix. Examples include Cr particles in a Cr2O3 matrixor Cr particles in an SiO2 matrix. In other implementations black chromecan be formed from a thin metal film of chromium upon which a thin filmof CrOx (a sub-oxide of chromium) has been either grown or deposited. Apreferred thickness for the black chrome is 150 nm.

The aperture windows 2709 can be patterned from the thin film stack ofmaterials 2704 and 2706 by processes known in the art such asphotolithography and etch or by photolithography and lift-off. In theetch process a layer of photoresist is added to the top of the thin filmstack and then exposed to UV light through a mask. After developing theaperture pattern in the exposed layer of photoresist, the whole stack isetched in the region of apertures 2709 down to the substrate 2702. Suchetching may be accomplished by immersion in wet chemicals, by a dryplasma or ion beam etch, or any combination of the above. In thelift-off process the layer of photoresist is added to the glass beforedeposition of the thin film stack, the resist being developed into apattern that is a reverse of the etch mask pattern. The thin film stackis then deposited over the top of the photoresist, such that the thinfilm stack makes contact to the glass everywhere except in the regionsof the apertures 2709. After deposition of the thin film stack iscomplete, the substrate is dipped into a bath of chemicals thatdissolves or lifts-off the photoresist as well as any thin filmmaterials that were deposited on top of the photoresist.

The spacer post 2708 is formed from a photo-imageable polymer such assuch as a photo-imageable epoxy (in particular a novolac epoxy) or aphoto-imageable polyimide material. Other polymer families that can beprepared in photo-imageable form and are useful for this applicationinclude polyarylene, parylene, benzocyclobutane, perfluorocyclobutane,silsequioxane, and silicone polymers. A particular photo-imageableresist useful for the spacer application is the Nano SU-8 materialavailable from Microchem Corporation, headquartered in Newton, Mass.

The polymer spacer material is initially deposited as a thick film ontop of the thin film stack 2704 and 2706 after the apertures 2709 havebeen patterned. The photo-imageable polymer is then exposed to UV lightthrough a mask. Alignment marks help to ensure that the resultantspacers 2708 are located correctly with respect to apertures 2709. Forinstance, alignment fiducials can be formed on the periphery of thedisplay during the process of etching the apertures 2709. Thesefiducials are then aligned to a corresponding set of fiducials on theexposure mask to ensure a correct location of spacers 2708. A developingprocess is then effective at removing all of the polymer except where itwas exposed to the UV light. In an alternate method, the features on theexposure mask may be aligned directly to display features on thesubstrate 2702, such as the apertures 2709.

In one implementation the spacer posts can be 8 microns tall. In otherimplementations spacer heights may range from about 2 microns to about50 microns, e.g., 4 microns. When cross sectioned in the plane of thesubstrate 2702, the spacers may take regular shapes such as a cylinderor a rectangle with widths in the range of 2 to 50 microns, e.g., 4microns. Alternately, they can have complex irregular cross sectionswhich are designed to maximize the contact area of the spacer whilefitting between other structures on the substrate, such as apertures2709. In a preferred implementation the spacer size, shape and placementis determined so that the spacers do not interfere with the movement ofthe shutters, such as shutters 406 or other MEMS components, such asactuators 404 in display apparatus 400.

In another embodiment, the spacer post 2708 is not provided as a polymermaterial but is instead composed of a heat re-flowable joining material,such as a solder alloy. The solder alloy can pass through a melting orre-flow step which allows the solder alloy to wet or bond to a matingsurface on the opposing substrate. The solder alloy therefore performsan additional function as a joining material between an aperture plateand a modulator substrate. Because of the reflow process, the solderalloy typically relaxes to an oblate shape referred to as the solderbump. A predetermined spacing between substrates can be maintainedthrough control over the average volume of material in the solder bump.Solder bumps can be applied to aperture plate 2700 by means of thin filmdeposition, by thick film deposition through a stencil mask, or byelectroplating.

In another embodiment, the aperture plate 2700 can be subjected to asandblasting treatment after the steps of forming the optical layers2704 and 2708. The sandblasting has the effect of roughening thesubstrate surface selectively in the regions of the aperture 2709. Aroughened surface at aperture 2709 behaves as an optical diffuser whichcan provide the benefits of a wider viewing angle for the display. Inanother embodiment, a diffusing surface at aperture 2709 is provided bymeans of an etching process, where the etch is selectively applied inthe regions of apertures 2709 after exposure of photoresist to aphotomask. Etch pits or trenches can be created through proper design ofthe photomask, and the sidewall angles or depths of the pits or trenchescan be controlled by means of either a wet or dry etch process. In thisfashion optical structures with controlled degrees of diffusivebroadening can be created. In this fashion anisotropic diffusers can becreated at the substrate surface which deflect light along a preferredoptical axis, creating elliptical and/or multi-directional cones ofemitted light.

In another embodiment, an etched trench can be provided in substrate2702 that substantially surrounds the display along the periphery of thearray of apertures 2709 (i.e. around the periphery of the active displayregion). The etched trench performs as a mechanical locating structurefor restricting the motion or flowing of an adhesive, such as adhesive528, used to seal aperture plate 2700 to an opposing substrate.

Further details regarding the materials and processes described abovecan be found in U.S. patent application Ser. No. 11/361,785, filed Feb.23, 2006, incorporated herein by reference. For example, thatapplication includes additional materials and processing methodologiesregarding the formation of dielectrically enhanced metal mirrors withapertures, light absorbing layers, and spacer posts. Although dielectricmirrors and spacers are described in that application in the context ofan integrated (for example MEMS-up) display design, it will beunderstood that similar processes can be adapted to the fabrication ofan aperture plate, such as aperture plate 2700.

In some implementations of the aperture plate 2700, it is desirable toemploy a transparent plastic material for the substrate 2702. Applicableplastics include, without limitation, polymethylmethacrylate (PMMA) andpolycarbonate. When plastic materials are used, it also becomes possibleto utilize an injection molding or stamping process for the formation ofspacer posts 2708. In such a process the spacer posts are formed in amold or a stamper first, before the application of the dielectricallyenhanced metal mirror 2704. All of the layers of the dielectricallyenhanced metal mirror 2704 would be then be deposited in sequence on topof the substrate which already includes spacer posts 2708. The lightabsorbing layer 2706 is deposited on top of the dielectric mirror 2704.In order to pattern the aperture window 2709 a special photoresist isapplied that uniformly coats the surfaces of the thin films withoutbeing disrupted by the presence of spacer posts 2708. Suitablephotoresists include spray-on photoresists and electroplatedphotoresists. Alternately, a spin-on resist is applied followed by areflow step that provides an even resist thickness across the thin filmsurfaces in the areas of apertures 2709. The exposure of the resist,developing, and etching of the thin film layers then proceeds asdescribed above. After the removal of the photoresist, the process iscomplete. A liftoff process can also be employed to pattern thedielectrically enhanced mirror as described above. The use of a moldingor stamping process for the formation of spacer posts 2708 helps toreduce the material costs required in the fabrication of aperture plate2700.

In some display implementations, the aperture plate, for instanceaperture plate 2804 is combined with a light guide, such as light guide516 into one solid body, referred to herein as a unitary or compositebacklight, described further in U.S. patent application Ser. Nos.11/218,690 and 11/528,191, respectively. Both applications areincorporated herein by reference. All of the processes described abovefor the formation of the dielectrically enhanced metal mirror 2704, forthe light absorbing layer 2706, and/or for the spacer posts 2708 can besimilarly applied to a substrate which is bonded to or otherwiseindistinguishable from the light guide. The surface of the unitarybacklight onto which the thin films are applied can be glass, or itcould be plastic, including a plastic which has been molded to formspacer posts, such as spacers 527.

In one implementation, the spacer posts 2708 are formed or attached toaperture plate 2700 before the aperture plate is aligned to a modulatorsubstrate. In an alternative implementation of display apparatus 500,the spacer posts 527 are fabricated on top of and as a part of themodulator substrate 504, before the modulator substrate is aligned to anaperture plate. Such an implementation was described with respect toFIG. 20 within the aforementioned U.S. patent application Ser. No.11/361,785.

FIG. 7 is a cross sectional view of a display according to anillustrative embodiment of the invention. The display assembly 2800comprises a modulator substrate 2802 and an aperture plate 2804. Thedisplay assembly 2800 also includes a set of shutter assemblies 2806 anda reflective aperture layer 2808. The reflective aperture layer 2805includes apertures 2810. A predetermined gap or separation between thesubstrate 2802 and 2804 is maintained by the opposing set of spacers2812 and 2814. The spacers 2812 are formed on or as part of themodulator substrate 2802. The spacers 2814 are formed on or as part ofthe aperture plate 2804. During assembly, the two substrates 2802 and2804 are aligned so that spacers 2812 on the modulator substrate 2802make contact with their respective spacers 2814.

The separation or distance of this illustrative example is 8 microns. Toestablish this separation, the spacers 2812 are 2 microns tall and thespacers 2814 are 6 microns tall. Alternately, both spacers 2812 and 2814can be 4 microns tall, or the spacers 2812 can be 6 microns tall whilethe spacers 2814 are 2 microns tall. In fact, any combination of spacerheights can be employed as long as their total height establishes thedesired separation H12.

Providing spacers on both of the substrates 2802 and 2804, which arethen aligned or mated during assembly, has advantages with respect tomaterials and processing costs. The provision of a very tall (e.g. 8micron) spacer, such as spacer 2708, can be costly as it can requirerelatively long times for the cure, exposure, and development of aphoto-imageable polymer. The use of mating spacers as in displayassembly 2800 allows for the use of thinner coatings of the polymer oneach of the substrates.

In another implementation, the spacers 2812 which are formed on themodulator substrate 2802 can be formed from the same materials andpatterning steps that were used to form the shutter assemblies 2806. Forinstance, the anchors employed for shutter assemblies 2806 can alsoperform a function similar to spacer 2812. In this implementation aseparate application of a polymer material to form a spacer would not berequired and a separate exposure mask for the spacers would not berequired.

Cell Assembly Methods

The assembly of a display module can comprise the alignment and bondingof two substrates. For instance, it is desirable to align a lightmodulator substrate, such as substrate 504 to a cover plate, such ascover plate 522. Alternatively, it is desirable to align a lightmodulator substrate, such as substrate 2806 to an aperture plate, suchas aperture plate 2804. FIG. 8 illustrates an alignment apparatus 3300for accomplishing the alignment process, according to an illustrativeembodiment of the invention. The alignment apparatus 3300 comprises astationary chuck 3302, a set of translational drives or motors 3304, avision system 3305, and a set of UV exposure lamps 3306. A modulatorsubstrate 3308 is rigidly attached to the chuck 3302. An aperture plate3310 is held in place and guided by the motors 3304. The motors 3304provide the ability to translate the substrate 3310 in threetranslational directions, for instance along x and y coordinates withinthe plane of substrate 3310 and additionally along the z coordinateestablishing and varying the distance between the two substrates 3308and 3310. Additionally, not shown in FIG. 8, an additional and optionalset of three rotational motors can be provided, which ensure both theco-planarity of the substrates 3308 and 3310 and also their properrotational relationship in the x-y plane. Although all translationalmotors 3304 are shown attached to the aperture plate 3310, in otherembodiments the motors can be arranged differently between the twosubstrates. For instance the x-y translation motors can be attached tothe aperture plate 3310 while the z-axis translation motor and thetarotation motor (about the z-axis) can be attached to the chuck 3302.

A variety of motor types are available for the motors 3304. In someembodiments these motors can be digitally controlled stepper motors, insome cases they can be linear screw drives, and in other cases they canbe magnetically-driven solenoid drives. The motors need not be arrangedto directly move a substrate, such as substrate 3310. They can insteadbe designed to move a stage or platter onto which the working piece orsubstrate 3310 is rigidly attached. The use of a moving stage isadvantageous, since an additional optical measuring system (in somecases a laser interference system) can be provided for the stage whichis capable of continuously measuring its translational position to aprecision of better than 1 microns. Feedback electronics can then beemployed between the motors 3304 and the optical measurement system toimprove both the accuracy and stability of the stage position.

In some embodiments of apparatus 3300 both the chuck 3302 and theoptional moving stage can be equipped with heaters and/or temperaturecontrol devices, to ensure uniform temperature across the substrates3308 and 3310. Uniform temperatures help to ensure proper alignmentbetween patterns on the two substrates, particularly for substrateswhose diagonals exceed about 20 centimeters.

The alignment apparatus 3300 incorporates a vision system 3305 fordetecting the relative positions of the two substrates 3308 and 3310. Ina preferred embodiment, alignment marks are patterned into thin films oneach of the substrates 3308 and 3310 (see, for example, the alignmentmarks 3408 and 3412 in FIG. 9. The vision system is capable ofsimultaneously imaging alignment marks on each of the two substrates,despite the fact that the marks are located on different surfaces, i.e.at different positions on the z axis.

For the illustrated embodiment, the vision system 3305 incorporates twoimaging lenses 3312 and 3313 and either a microscope capable ofsplit-field imaging or two cameras 3314 and 3315. The vision system 3305is therefore capable of imaging, substantially simultaneously, twoseparated sets of alignment marks. The two sets of alignment marks arepreferably located at the far sides or corners of the modulation arrayor panel.

In operation, an operator uses the vision system 3305 to view therelative positions of alignment marks, such as marks 3408 and 3412, andthereby judge the direction and degree of misalignment between the twosubstrates. The operator can then adjust the alignment betweensubstrates 3308 and 3310, using drive motors 3304, until the alignmentmarks on the two substrates indicate misalignment below an acceptabledegree of error. After sufficiently reducing the misalignment, theoperator drives the z-axis motor until the spacers, such as any of thespacers 1010, on one of the substrates, 3308 or 3310, contact theopposing substrate, 3308 or 3310, or opposing spacers. In manyinstances, due to mis-orientation or non-planarity of the substrates,the operator will need to continually refine the x-y alignment betweenthe substrates as the z-axis distance between the two substrates isdecreased. In some embodiments, a final x, y, and theta correction canbe made even after contact is established between the substrates.

After contact is made, an adhesive 3318 will also make contact betweenthe two substrates. In some embodiments, as the last step in the method3301, the adhesive is at least partially cured while the alignmentapparatus 3300 holds the two substrates in position. The UV exposurelamps 3306 can be used to initiate or accelerate the curing of theadhesive, thereby bonding the two substrates together. In someembodiments the substrate stage or the chuck 3302 is equipped withheaters to affect a thermal curing of adhesive 3318. The alignmentmarks, e.g. marks 3408 and 3412, are usually patterned and etched at thesame time and are printed from the same photomask as the masks used topattern the apertures. The alignment marks are therefore designed for afiduciary marker function, i.e. the operator who achieves sufficientalignment between the alignment marks has confidence that the shuttersand apertures in the neighboring array will also be in properly aligned.

According to the discussion of display apparatus, the overlap ispreferably greater than or equal to 2 microns. In practice an overlapW2, which is reliably achieved during manufacture, is determined by asafety margin, designed into the masks, and by an alignment precision ortolerance. The precision or achievable tolerance is based on the designof alignment apparatus 3300, the design of the alignment marks, andprocess variables such as temperature, pressure, and the viscosity orplasticity of seal materials. Two examples are provided below foracceptable tolerance design: In the first example, which is tolerant ofrelatively wide variations in alignment during manufacture, an array ofshutters and apertures is designed with a nominal overlap of 22 microns,i.e. if perfectly aligned, the shutters are designed to overlap theapertures by 22 microns. If the apparatus 3300 then enables an alignmentrepeatability of .+−. 20 microns, the designer can be assured that all(or 99.99% depending on how reliability is specified) of the shutterswill have an overlap of at least 2 microns. However, for a dense arrayof pixels, i.e. for a high resolution display, there is not usually roomavailable in an array design for 22 microns of overlap. Therefore a moreprecise alignment capability is desired.

In the second example, a nominal overlap of only 1 microns is providedfor in the masks, and the apparatus 3300 is designed to provide analignment precision within .+−. 1 microns between patterns on the firstand second substrates. To achieve this precision a) the vision system3305 a resolution smaller than 1 microns, b) the motors 3304 (orassociated translation stages) stably drive to and resolve positiondifferences with a resolution smaller than 1 microns, and c) thealignment marks are patterned and etched with edges, dimensions, and/orplacements that are precise to a resolution of better than 1 microns.Automated alignment systems with sub-micron precision are availabletoday for purposes of semiconductor mask alignment, optoelectroniccomponent assembly, and micro-medical devices. Representative suppliersof these systems include the Automation Tooling Systems Corp. ofCambridge, Ontario, Canada and the Physik Instrumente LP of Karlsruhe,Germany.

Generally, if attention is paid to the design of the vision system, thedrive motors, and the design of the alignment marks, then it possible toprovide an alignment apparatus 3300 and an alignment method which iscapable of ensuring an overlap between shutters and apertures that isgreater than 0 microns and less than 20 microns. In a preferred design,the alignment method is capable of ensuring and overlap that is greaterthan 0 microns and less than 4 microns.

The alignment method described above was provided as one example of analignment method that assigns active control of the motors 3304 to ahuman operator. In other methods the intervention of an operator is notrequired to achieve alignment. Intelligent vision (machine vision)systems are available, for example, from the vendors identified above,for the apparatus 3300 (i.e. systems which include digital cameras andcomputer image processing) that can measure the direction as well-as theamount of misalignment between fiducials on two substrates and then canautomatically drive the motors 3304 until the measured misalignmentbecomes less than a pre-specified level.

The alignment marks or fiducials employed by apparatus 3300 can takemany forms, other than those shown or discussed with respect to FIG. 9below. In some embodiments the operator or the machine vision system iscapable of recognizing specific functional patterns on the substrates,such as the shapes of shutter assemblies or apertures. The vision systemthereby measures and minimizes directly the misalignment betweenshutters and apertures. In another embodiment, the display edges are cutor diced to a precise position with respect to the positions of theshutters and apertures. The vision system thereby measures and minimizesthe misalignment between the edges of the two substrates.

After either a human operator or the automatic alignment system bringsthe substrates into alignment and establishes contact between the twosubstrates, the UV exposure lamps 3306 can be employed to at leastpartially cure the adhesive 3318. The adhesive bonding material 3318prevents the subsequent relative movement between substrates 3308 and3310 after alignment has been achieved in apparatus 3300. Alternatemeans are available for maintaining alignment between the two substratesafter alignment. These alternate means include the use of alignmentguides, such as alignment guides, and heat reflowable spacer materialssuch as spacer.

Although the functioning of alignment apparatus 3300 was with theexample of display 500 in the MEMS-down configuration, similar alignmenttechniques can be useful when applied to the MEMS-up configuration, asillustrated by display apparatus 500. In display assembly 500 theshutter assemblies 502 are formed on substrate 504 while the blackmatrix and associated apertures 524 are formed on substrate 522. The twosubstrates 504 and 522 can be aligned using alignment apparatus 3300such that an overlap exists between at least one edge of the shutters503 and the edge of a corresponding aperture in black matrix 524. Thealignment apparatus 3300 ensures an overlap between edges of between 0and 20 microns. In a preferred design, the alignment method ensures anoverlap that is greater than 0 microns and less than 5 microns, or insome cases, less than 4 microns.

Although the functioning of alignment apparatus 3300 was described for adisplay incorporating transverse-shutter-based light modulators, such asa shutter assembly, it will be understood that the alignment apparatus3300 and alignment method described above can be usefully applied toalternate MEMS light modulator technologies. For instance, theelectrowetting modulator array benefits when the aperture plate isaligned to the modulator substrate such that an overlap is establishedbetween the edge of the oil and the edge of apertures in thelight-obstructing, filtered, or dark state. Similarly rolling actuatorlight modulators, such as light modulator 220 can be fabricated andaligned in similar fashion, wherein an overlap is provided between thelight obstructing edge of the roller-actuator-modulator on a firstsubstrate and the edge of a corresponding aperture which has beenpatterned on a second substrate.

Other non-shutter-based modulators can benefit from the alignmentapparatus 3300 and method described above. For instance, a MEMSinterference modulator or a MEMS light tap modulator, such as lightmodulator 250, fabricated on a first substrate can be aligned to theedge of a black matrix fabricated on a second substrate. Details ofthese light modulators can be found in U.S. Pat. Nos. 6,674,562 and5,771,321, incorporated herein by reference.

Panel Fabrication Processes

Manufacturing productivity is increased whenever the modulator arraysfor multiple displays can be built in parallel fashion on the same glassor plastic substrate. Large glass substrates, referred to as panels, andassociated fabrication equipment, are now available in sizes up to 2meters square. FIG. 9 illustrates how multiple arrays of MEMS lightmodulators can be formed onto one large modulator substrate 3402 whilemultiple arrays of aperture holes can be formed on a large apertureplate 3404, according to an illustrative embodiment of the invention.The panel 3402 includes a set of 6 modulator arrays 3406 plus a set offour modulator alignment marks 3408. The panel 3404 includes a set ofsix aperture arrays 3410 plus a set of four aperture alignment marks3412. Each of the modulator arrays 3406 is designed to correspond to oneof the aperture arrays 3410, such that when the panels 3402 and 3404 arealigned and sealed together, the corresponding modulator array-aperturearray pairs will each form a display assembly, also referred to as acell assembly. A single alignment and sealing operation betweensubstrates 3402 and 3404, then, suffices to simultaneously align andseal 6 cell assemblies. For the example shown in FIG. 9, the glasspanels 3402 and 3704 are 30 cm in diagonal while each of the cellassemblies or display areas would be 10 cm in diagonal. In otherembodiments, panels as large as or larger than 50 cm in diagonal may beemployed to fabricate up twenty five 10 cm diagonal displays per panel.

Also shown are the epoxy adhesive lines (one type of seal material)3414, and spacer posts 3416 added to each of the arrays on the apertureplate 3404. A variety of spacers are applied to the interior of eacharray on aperture plate 3404, as described with respect to displayassemblies. The process for applying the adhesive will be describedbelow with respect to the cell assembly step 3614.

A panel assembly 3500, after completion of alignment and seal of panels3402 and 3404, is illustrated in the FIG. 10, according to anillustrative embodiment of the invention. Successful alignment of thetwo substrates is indicated by the nesting of the modulator alignmentmarks 3408 within the aperture alignment marks 3412. The alignment markscan be designed such that a nominal 1 microns gap is allowed between theinner edge of the mark 3412 with the outer edge of the mark 3408 (themagnitude of these gaps is exaggerated in FIG. 10 for purposes ofillustration). In this alignment design, the operator and/or automaticalignment system adjusts the relative position of the substrates in thetool 3300 until appropriate gaps are visible in both the x and ydirections for the nested alignment marks, e.g., until none of the linesare crossed or touching. When the appropriate gaps are visible thealignment is considered successful, i.e. misalignment has been reducedto within an acceptable error and the expected overlap betweenmodulators and apertures in each of the arrays 3406 and 3410 has beenachieved.

The nominal gap between alignment marks can be designed to match theanticipated precision of the alignment process, e.g. the gap can be 10microns, 4 microns, 2 microns, or 1 micron depending on the alignmentprecision desired. In an alternate design, one alignment mark is acircular dot while the other mark is shaped as a ring. A gap can bedesigned between the dot and the ring corresponding to the desiredalignment precision. In some alignment machine designs, a gap betweenalignment marks is not required; instead the machine uses a digitalcamera to estimate the center points of both dot and ring. The alignmentsoftware then seeks to align the center-points of dot and ring. The twopanels 3402 and 3404 are bonded in place by an adhesive. The curing ofthis adhesive is described below with respect to the cell assembly step3620.

FIG. 10 also illustrates a set of dicing lines 3502 superimposed uponthe panel assembly 3500. The dicing lines 3502 mark the lines alongwhich the panel will be cut so that individual arrays, also referred toas displays or cell assemblies, can be separated from panel. Theseparation process, also referred to a singulation, can be accomplishedby means of a scribe and break method. In this process a diamond orcarbide tip is used to scratch a line along the surface of the glasspanels at lines 3502. A simple bending process can then be used to breakthe panels along the scribe lines. In another embodiment the separationor singulation process is accomplished by means of a dicing saw. It isnot necessary that both substrates 3402 and 3408 be cut along the samedicing lines. It is often advantageous that the modulator substrate bediced to a perimeter width that is wider than that prescribed for theaperture substrate. This allows room for the bonding of driver chips,after cell assembly is complete, on the edge of the modulator substrate.At times, such as when dual fill holes are used, the panel is separatedinto strips by cutting it along the axis 3504.

FIG. 11 illustrates a first method 3600 for assembling a displayapparatus (also referred to as cell assembly method 3600) incorporatingMEMS light modulators, according to an illustrative embodiment of theinvention. A first embodiment of method 3600 will be described withrespect to a MEMS-down display assembly. A second embodiment, forassembly of displays in the MEMS-up configuration, will be describedthereafter.

The cell assembly method 3600 for MEMS-down displays begins withprovision of two substrates at steps 3602 and 3604. Both of thesesubstrates are transparent, made from glass or plastic. The assemblymethod continues with the fabrication of a control matrix, at step 3606,and the fabrication of the MEMS modulator array, at step 3608. In oneembodiment both the control matrix and the modulator array arefabricated onto the first substrate, referred to as the modulatorsubstrate. In one embodiment, a trench, also known as a bubble trappingregion is provided in the modulator substrate. As discussed with respectto display assembly 3100, however, there are embodiments where thecontrol matrix can be fabricated on a substrate distinct from themodulator substrate and be electrically connected to it by means ofelectrically conductive spacers. Further detail on the fabrication ofthe modulator substrate can be found in the U.S. patent application Ser.No. 11/361,785 referenced above.

The MEMS-down assembly method 3600 proceeds at step 3610 with thefabrication of an aperture layer. The aperture layer is fabricated ontothe second substrate, which is preferably made of a transparentmaterial, e.g. plastic or glass. In the MEMS-down configuration thesecond substrate is referred to as the aperture plate. In oneembodiment, a trench, also known as a bubble trapping region is providedin the surface of the aperture plate at step 3611. In other MEMS-downembodiments the second substrate, on which the aperture layer isfabricated, is also utilized as a light guide. In some embodiments, theaperture layer is composed of a light absorbing material which ispatterned into a series of apertures. In one embodiment, the aperturelayer is designed to reflect light incident from the substrate backtoward the substrate.

The method continues with the application of spacers (step 3612) andsealing materials (step 3614) to one or the other of the two substrates;the substrates are then aligned and bonded together. The method 3600continues at step 3612 with the application of spacers. Any of thespacers illustrated by the spacers 2708, 2812, or 2814 including thefabrication methods described therefore can be incorporated at step3612. The spacers may be formed onto either or both of the first andsecond substrates.

The method 3600 continues at step 3614 with the application of a sealmaterial, such as the epoxy seal material 528. The seal material can beapplied to either or both of the first and second substrates employed bythe method 3600. The seal material is an adhesive bonding material,which will maintain the position of the first and second substratesafter the alignment step. The seal material is also used to contain thefluid, to be added at step 3624, within the gap between the twosubstrates. Applicable seal materials can be a polymer material such asan epoxy, an acrylate, or a silicone material or the seal material canbe formed from a heat-reflowable solder metal such as solder bump.

In some embodiments the seal material can be a composite material, suchas the anisotropic conductive adhesive 3214. The seal material can bedispensed from a nozzle, which moves along the periphery of each of themodulator or aperture arrays, as shown for display panel 3404 in FIG. 9.

The seal material 3414 does not completely encircle the periphery ofeach display area on display panel 3404. One or more gaps 3418, referredto as the filling holes, are intentionally left in the peripheral sealto accommodate the filling of the cell with fluid at step 3624. In oneembodiment, these may be on opposite sides, along the same side, or inany side. In some embodiments, this gap is left open next to a bubbletrapping region, so that a bubble can be intentionally introduced into aspace enclosed by the seal.

The method 3600 continues at step 3616 with the optional dispense of aconductive adhesive. If the spacers or the seal material added at steps3612 and 3614 do not have a conducting property, then it is oftenadvantageous to add an additional adhesive with this property. Theconductive adhesive added at step 3616 allows for an electricalconnection between the control matrix on the first substrate and theaperture layer on the second substrate. The adhesive added at step 3616is usually located at some point along the periphery of the displayarea.

After dispense, a seal material undergoes a cure step to becomerelatively hard and rigid. Although seal material may be partially curedas part of the step 3614, in many embodiments a final cure does notoccur until one of the later steps 3618 or 3620. The seal material maybe formulated to allow for many alternate types of curing, includingdesiccation curing, UV or ultraviolet curing, thermal curing, ormicrowave curing. When employing an alignment tool, such as theapparatus 3300, a UV-cured epoxy can be preferred.

As indicated in FIG. 11, the steps for fabrication of the control matrix3606, fabrication of MEMS modulators 3608, fabrication of the aperturelayer 3610, application of spacers 3612, and application of sealmaterial 3614 can all be performed at the panel level where multipledisplays are fabricated simultaneously on a large glass or plasticpanel. Alternately, these steps may be performed for individual displayson smaller substrates. Further fabrication details for assembly steps3606, 3608, 3610, and 3612 can be found in the U.S. patent applicationSer. No. 11/361,785, referenced above.

The method 3600 continues at step 3618 with the alignment of the firstand second substrates, as was described with respect to the alignmentapparatus 3300 in FIG. 8. The alignment apparatus 3300 includes a cameraand/or microscope system for confirming that the alignment is accurateto within an acceptable degree of error. The first and second substratesare brought into contact by means of the spacers as part of thealignment step 3618.

As part of the alignment step 3618 the adhesive bonding material is atleast partially cured to bond or maintain the relative positions of thetwo substrates. The alignment apparatus 3300 includes heaters and/or UVexposure lamps to affect cure of the adhesive. In some embodiments thewhole perimeter seal, such as seal 3414, is at least partially cured aspart of step 3618. In other embodiments a plurality of uv-curableadhesive dots is provided on the substrates prior to alignment, inaddition to a thermally-curable seal material 3414. For this embodimentonly the epoxy dots are cured as part of the alignment step 3618, whilethe remainder of the seal material is cured later, at step 3620.

The method 3600 continues at step 3620 with the cure of the sealmaterial. In many embodiments the proper alignment between first andsecond substrates can only be maintained when the seal material behavesas a relatively rigid adhesive. The adhesive cure at step 3620 ensuresthe rigidity of the seal. The cure at step 3620 can be carried out byeither a thermal, UV, or a microwave cure. In some embodiments the sealis cured at step 3620 by placing the assembly into an oven, or UV ormicrowave exposure system, under pressure or between the plates of apress. The press helps to minimize bending or warping in the substrateswhile the adhesive is being cured. The press helps to ensure that thegap are maintained by ensuring a rigid contact of each substrate to thespacers.

The method 3600 continues at step 3622 with the optional separation ofindividual display arrays from a large panel containing multiple arrays.Such separation is only required if the cell assembly steps, up untilthis point, have proceeded according a large panel process, as describedin FIG. 9. If the modulation substrate and aperture plates arefabricated as individual displays at steps 3606 to 3614, then nosingulation or separation step is necessary. The separation may beaccomplished by either a scribe and break process or by a dicing saw.

The method includes the separation or singulation of individual displaysfrom a larger panel assembly (step 3622) and the filling of the gapbetween the two substrates with a fluid or lubricant (step 3624),filling the display assembly with fluid. As indicated in the discussionsof display apparatus 500, the two substrates of a display apparatus arepreferably separated by a gap, such as the gap 526, and the gap isfilled by a fluid, such as working fluid 530. For many displays thefluid acts as a lubricant which substantially surrounds the MEMS lightmodulators. The fluid also has defined electrical and optical propertiesas discussed above. In one embodiment, one or more of the filling holeswill be sealed at step 3626. Subsequently, a bubble is intentionallyintroduced within a bubble trapping region at step 3625. In anotherembodiment, any and all filling holes are sealed after the bubble isinduced into the bubble trapping region at step 3628.

The cell assembly method 3600 will now be reviewed for its applicationto the MEMS-up display configuration, examples for which are given bydisplay assembly 500 of FIG. 5. For the MEMS-up display configurationboth the control matrix and the MEMS modulator array are fabricated onthe first substrate at steps 3606 and 3608. Examples are given asmodulator substrates. An aperture layer is deposited on the secondsubstrate at step 3610.

As discussed with respect to display assembly 3100 there are embodimentswhere the MEMS modulator array is fabricated on the first substratewhile the control matrix can be fabricated on the second substrate. Thetwo substrates are in electrical communication by means of conductivespacers.

For the MEMS-up display configuration the second substrate is referredto as a cover plate, such as cover plate 522. The aperture layer,fabricated at step 3610, is referred to as a black matrix layer, such asblack matrix 524, and is patterned into an array of apertures. The blackmatrix layer is preferably comprised of a light absorbing material toimprove the ambient contrast of the display. After assembly, the blackmatrix apertures preferably overlap the MEMS light modulators which arelocated on the modulator substrate.

For the MEMS-up display assembly method 3600 the cover plate, i.e. thesecond substrate provided at step 3604, is preferably made of atransparent material, i.e. plastic or glass. For the MEMS-up assemblymethod, however, the modulator substrate provided at step 3602 can bemade from an opaque material, such as silicon. For instance, for areflective MEMS-up display, the first substrate, e.g. silicon, can becoated with a reflective layer at one of steps 3606 or 3608. For atransmissive MEMS-up display, an opaque material employed for the firstsubstrate can be etched with an array of through-holes at the positionsof apertures, such as apertures 508.

For the MEMS-up display assembly 3600, spacers are applied at step 3612,and seal material is applied at step 3614 to either of the first orsecond substrates, i.e. either the modulator substrate or the coverplate. As with the case of the MEMS-down, the seal material is appliedcompletely around the periphery of the space enclosed by the seal,leaving one or more openings that will be later sealed in a fashionsimilar to that described above for MEMS-down.

The subsequent steps in a MEMS-up display assembly method 3600 aresimilar to the MEMS-down display assembly method 3600, including thealignment step 3618, the cure of seal material, step 3620, theseparation of multiple displays from the panel, step 3622, fluid fillingat step 3624, as well as final sealing step 3626 and 3628.

As described with respect to the alignment apparatus 3600, the assemblymethod 3600 in either the MEMS-up or the MEMS-down configuration isapplicable to a number of alternate MEMS light modulator technologies,including electrowetting displays and rolling-actuator displays. TheMEMS-up display assembly method 3600 is particularly applicable tointerference modulator displays and MEMS light tap modulator displays.

The details of the fluid filling process (step 3624) will be describedwith respect to the fluid filling apparatus 3700 which is illustrated inFIG. 12, according to an illustrative embodiment of the invention. Thefluid fill apparatus is formed from a vacuum chamber 3702 which ispartially filled with a reservoir of the working fluid 3704. An alignedand partially sealed cell assembly or panel assembly, such as panelassembly 3500, is suspended above the fluid reservoir by a wand 3706, oralternately by a moveable platter. Attached to the vacuum chamber is aport 3708 leading to a vacuum pump and a port 3710 used to vent theinterior of the vacuum chamber to atmospheric pressure. Valves areassociated with each of the ports 3708 and 3710, although not shown inFIG. 12.

In operation, the process for filling the gap between the substrates ina panel assembly, such as assembly 3500, is a two-step process. Firstthe air or other gas is removed from between the two plates and, second,the gap is filled by the fluid. Air is removed from between the plateswhen the valve to the vacuum pump is opened and the whole chamber 3702is reduced to a pressure substantially below 1 torr. Next the vacuumvalve is closed and the wand 3706 is used to immerse the panel assembly3500 into the reservoir 3704 of the working fluid. Once immersed, thevent valve is opened to the air, or to clean nitrogen or argon gas froma bottle. The returning air brings the pressure on all fluids back toatmospheric pressure (or pressures greater than 600 torr). Underpressure, the working fluid is then forced into the gap between thesubstrates of cell assembly 3500. When the cell assembly is removed fromthe chamber 3702 the cell assembly is filled by the fluid, thuscompleting the assembly step 3624.

In an alternate design, the panel assembly 3500 need not be suspended bya moveable wand, such as wand 3706. Instead the panel assembly can befixed in place and the lubricant 3705 can be moved into or out of thevacuum chamber 3702 by means of a series of valves. The chamber isevacuated while fluid is largely absent from the chamber. Afterevacuation, the fluid level in the chamber is increased by flowingadditional fluid into the chamber. Fluid is added until the assembly3500 is immersed in fluid. After immersing the panel in fluid the systemis vented to atmospheric pressure to fill the gap with fluid.

In another embodiment the chamber 3702 is not filled with a liquid, asin liquid 3704, but is instead backfilled with a gas after evacuatingthe chamber. Examples of backfill gases include inert gases (argon,nitrogen), vapor phase lubricants, certain reactive gases, or anycombination of the above. The reactive gases are those that can reactwith or be deposited onto the moving surface of the MEMS modulators.They can chemically pacify or reduce the stickiness of moving surfacesby reducing its surface energy. Examples include, without limitation,dimethyldichlorosilane (DDMS),tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane (FOTS), andheptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane (FDTS). Vaporphase lubricants are gases that remain in the vapor phase substantiallythroughout the operation of the device, and are similarly capable ofpacifying surfaces. Examples include, without limitation, sulphurhexafluoride, and the vapor phase forms of methanol, ethanol, acetone,ethylene glycol, glycerol, silicone oils, fluorinated silicone oils,dimethylsiloxane, polydimethylsiloxane, hexamethyldisiloxane, anddiethylbenzene, or any mixture of the above.

The fluid filling process within chamber 3702 can be executed on eitherthe cell level or the panel level. In assembly process 3600 thesingulation step 3622 precedes the fluid filling step 3624, meaning thatthe cell assemblies for individual displays are loaded into vacuumchamber 3702 for fluid filling. The vacuum fill chamber 3702 can includea platter capable of holding and immersing multiple individual displaysin a single pump-down operation—so that multiple displays can be filledwith fluid at the same time. Alternately it is possible to reverse theorders of these steps and load a complete panel assembly, such asassembly 3500, into the vacuum chamber. The gaps within each of thedisplays on the panel are then evacuated and filled at the same time.The dicing or singulation process 3622 then occurs after the fluidfilling step 3624 is complete.

A cell assembly is completed in the method 3600 with the sealing of thefilling hole, at step 3626. The fluid was allowed or forced into thespace between substrates at step 3624 through the filling hole 3418 thatwas left in the perimeter seal at step 3614. This hole is filled with anadhesive at the end of the assembly process to prevent leakage of thefluid out of the display assembly. As part of step 3626 and prior tosealing the fill hole, pressure can be applied to the cell via a press.The press compresses the two substrates, forcing the spacers fabricatedon one substrate, into intimate contact with the other substrate. Thisestablishes a uniform gap or spacing between the two substrates. Thefill hole 3418 is then sealed with adhesive prior to removal of pressurefrom the display. Once sealed, the closed and fluid-filled chamberwithin the cell prevents the substrates from separating under ambientconditions. The adhesive employed at step 3626 may be a polymer adhesivethat is cured using either thermal curing, UV curing, or microwavecuring.

The cell assembly method 3600 will now be reviewed for its applicationto the MEMS-up display configuration, an example of which is given bydisplay assembly 500. For the MEMS-up display configuration both thecontrol matrix and the MEMS modulator array are fabricated on the firstsubstrate at steps 3606 and 3608. Examples are given as modulatorsubstrates 504 or 2418. An aperture layer is deposited on the secondsubstrate at step 3610.

As discussed with respect to display assembly 3100 there are embodimentswhere the MEMS modulator array is fabricated on the first substratewhile the control matrix can be fabricated on the second substrate. Thetwo substrates are in electrical communication by means of conductivespacers.

For the MEMS-up display configuration the second substrate is referredto as a cover plate, such as cover plate 522. The aperture layer,fabricated at step 3610, is referred to as a black matrix layer, such asblack matrix 524, and is patterned into an array of apertures. The blackmatrix layer is preferably comprised of a light absorbing material toimprove the ambient contrast of the display. After assembly, the blackmatrix apertures preferably overlap the MEMS light modulators which arelocated on the modulator substrate.

For the MEMS-up display assembly method 3600 the cover plate, i.e. thesecond substrate provided at step 3604, is preferably made of atransparent material, i.e. plastic or glass. For the MEMS-up assemblymethod, however, the modulator substrate provided at step 3602 can bemade from an opaque material, such as silicon. For instance, for areflective MEMS-up display, the first substrate, e.g. silicon, can becoated with a reflective layer at one of steps 3606 or 3608. For atransmissive MEMS-up display, an opaque material employed for the firstsubstrate can be etched with an array of through-holes at the positionsof apertures, such as apertures 508.

For the MEMS-up display assembly 3600, spacers are applied at step 3612,and seal material is applied at step 3614 to either of the first orsecond substrates, i.e. either the modulator substrate or the coverplate.

The subsequent steps in a MEMS-up display assembly method 3600 aresimilar to the MEMS-down display assembly method 3600, including thealignment step 3618, the cure of seal material, step 3620, theseparation of individual displays from the panel, step 3622, and fluidfilling at step 3624.

As described with respect to the alignment apparatus 3600, the assemblymethod 3600 in either the MEMS-up or the MEMS-down configuration isapplicable to a number of alternate MEMS light modulator technologies,including electrowetting displays and rolling-actuator displays. TheMEMS-up display assembly method 3600 is particularly applicable tointerference modulator displays and MEMS light tap modulator displays.

FIG. 13 illustrates an alternative method for assembling a displayapparatus incorporating MEMS light modulators. The method 3800 can beemployed for assembling displays into either the MEMS-down configurationor the MEMS-up configurations as described with respect to the method3600. Similar to method 3600, the assembly method 3800 begins with theprovision of two substrates (steps 3802 and 3804) upon which the displaycomponents are fabricated (steps 3806, 3608, and 3810). The method 3800continues with the application of spacers (step 3812) and sealingmaterials (step 3814) to one or the other of the two substrates. Themethod 3800 also includes filling the gap in a display assembly withfluid (step 3118). However, in contrast to the method 3600, the order offluid filling (step 3818) and display assembly (steps 3820, 3822, and3824) are reversed. The assembly method 3800 is sometimes referred to asthe one-drop fill method.

The assembly method 3800 begins with the provision of first and secondsubstrates, steps 3802 and 3804 and continues with the fabrication ofthe control matrix (step 3806), the fabrication of a MEMS modulatorarray (step 3808), the fabrication of an aperture layer (step 3810), andthe application of spacers (step 3812). These steps comprisesubstantially the same assembly processes as were used for correspondingsteps in the assembly method 3600.

The method 3800 continues at step 3814 with the application of a sealmaterial. Similar seal materials and similar dispense methods can beapplied at step 3814 as were used for the seal application step 3612.For the step 3814, however, no gap or filling hole is left in the sealmaterial around the periphery of the active areas of the display.

An optional conductive adhesive application, step 3816, follows which issimilar to the adhesive application step 3616.

The method 3800 continues at step 3818 with the application of a liquid.The applicable liquids, comprising lubricating and other electrical,mechanical, and optical properties were described above with respect todisplay apparatus 500. For a liquid filling step (step 3800), a vacuumfilling apparatus, such as apparatus 3700 is not necessary. A correctvolume of fluid can be dispensed directly onto the open face of one ofthe first or second substrates. The fluid is preferably dispensed on thesecond substrate on which the aperture layer is formed, as thissubstrate generally has no MEMS components with cavities or re-entrantsurfaces where air bubbles might collect. When the substrates are largepanels incorporating multiple displays, as in panel 3404, a correctvolume of fluid is dispensed into the active area of each array.Generally, the fluid will spread over the face of the substrate untilconfined by the perimeter of seal material 3414. A correct volume offluid will completely fill the cavity defined by the perimeter seal. Insome embodiments an optical measurement tool is used to determine anaccurate volume for each cavity before filling—by measuring actualperimeter dimensions for individual arrays on the panel.

The method 3800 continues at step 3820 with the alignment of the twosubstrates. As the lubricating fluid has already been applied to one ofthe substrates, the alignment apparatus required for step 3800 willdiffer from that shown by apparatus 3300. As a primary difference, thealignment operation is preferably carried out under reduced pressure orvacuum conditions. This means that the first and second substrates, aswell as many of the moving parts and parts of the vision system providedfor alignment are now operated within a vacuum chamber, referred to asan alignment chamber.

In operation the two substrates would be introduced to the alignmentchamber and the chamber would be evacuated. To prevent evaporation ofthe fluid (already dispensed as at step 3818), the chamber may bebackfilled to an equilibrium vapor pressure of the lubricating fluid.After the two substrates are aligned and brought together, thelubricating fluid will make contact with the surface of each of the twosubstrates and will substantially surround each of the moving parts ofthe MEMS light modulators. After the substrates are in contact and thefluid has wet all surfaces, the alignment chamber is vented back toatmospheric pressure (at step 3822). A partial cure can also be appliedto the adhesive after the substrates have been brought into contact. Theadhesive can be cured by either thermal means or by means of UV lampsinstalled within the vacuum chamber.

In some embodiments, where the lubricating fluid has a high vaporpressure, i.e. where it can evaporate quickly at ambient temperatures,it may not be practical to dispense the fluid at step 3818 before thepanels are introduced into the alignment chamber. For this embodiment,after the alignment chamber has been evacuated and backfilled with avapor pressure of the lubricant, a nozzle can be provided whichdispenses the lubricating fluid onto one of the substrates immediatelybefore the step of aligning of the two substrates.

To the extent that the seal material was not completely cured during thealignment operation at step 3820, a further curing step is applied atstep 3824. The cure at step 3824 can be carried out as either a thermal,a UV, or a microwave cure.

The method 3800 is completed at step 3826 with the optional separationof individual display arrays from a large panel containing multiplearrays. Such separation is only required if the cell assembly steps upto this point have proceeded according a large panel process, asdescribed in FIGS. 9 and 10. If the modulation substrate and apertureplates where fabricated in the form of individual displays at steps 3806to 3814, then no final separation step is necessary. The separation maybe accomplished by means of either a scribe and break process or by useof a dicing saw.

The final steps for assembling a display, after completion of the method3600, are often referred to collectively as the module assembly process.The module assembly incorporates the steps of attaching a silicon chipor chips comprising control and drive circuitry directly to the glasssubstrate, bonding flexible circuits for interconnecting the display toexternal devices, bonding optical films such as contrast filters,affixing a backlight, and mounting the display into a support structureor enclosure. The flexible circuit may be comprised of simpleinterconnects or may contain additional electrical elements such asresistors, capacitors, inductors, transistors or integrated circuits.

FIG. 14 illustrates an alternative method 4000 for assembling a displayapparatus, according to an illustrative embodiment of the invention. Themethod 4000 is representative of a cold seal process which helps toprevent and avoid the formation of vapor bubbles during operation of thedisplay. The assembly method 4000 is similar and includes many of thesame process steps as assembly method 3600, except that in method 4000the final sealing of the fluid within the cavity is carried out atreduced temperatures.

The assembly method 4000 comprises, inter alia, steps for fluid filling,cell cooling, cell compression, and cold seal. A MEMS display cell,including a first substrate and a second substrate defining a cell gapare provided at step 4002. Each of the substrates is transparent. Thefirst substrate may contain an array of light modulators. The secondsubstrate may comprise either a cover plate, such as cover plate 522, oran aperture plate, such as aperture plate 2804.

In the display cell, the second substrate is positioned adjacent to thefirst substrate such that the first substrate is separated from thesecond substrate by a gap. The first substrate and the aperture platemay be formed from any suitable material, e.g., plastic or glass. Aplurality of spacers is formed between the first and second substrates(step 4004) which maintain a substantial portion of the gap. The spacersmay be formed on either of the first and second substrates. In oneembodiment, similar to display assembly 2800, the spacers are formed onboth of the first and second substrates. Any of the materials orfabrication methods described with respect to aperture plate 2700 andthe spacers 2708 without limitation may be applied for this purpose.

The assembly method 4000 continues with the application of an edge seal(step 4006). The edge seal is applied around at least a portion of theperimeter of the array of light modulators. The seal material isprovided as an adhesive for the bonding together of the first and secondsubstrates. Any of the seal materials, including epoxy adhesives,described with respect to step 3614 without limitation may be employedfor this purpose. In some embodiments, the seal is cured after thedisplay assembly is filled. The substrates may be aligned, as describedabove, before the seal is cured. At least one fill hole is provided inthe perimeter of the edge seal to allow for the filling of a fluid intothe gap. In one embodiment, the seal material is a composite materialthat includes spacer materials within it. The included spacer materialsmay be made of plastic, glass, ceramic or other material. A sealmaterial that is suitable for this purpose is a UV curable epoxy sold bythe Nagase Chemtex Corporation with the product name XNR5570. The sealmaterial XNR5570 can be enhanced to provide a spacer function by mixingin a concentration of one or more 12 micron radius glass spheres. Thespacers may be incompressible. Suitable microstructures for the spacersinclude beads or spheres, although other shapes and microstructures canbe suitable without limitation. In some embodiments, the spacer heightestablished by the seal material is substantially larger than the heightof each of the spacers, e.g. spacers 2708, located within the displaycell.

The display assembly is then filled, via the fill hole, with a workingfluid at step 4008. The filling may occur in a fluid fill chamber, andwith a filling process similar to that described with respect to fillingapparatus 3700. The working fluid substantially surrounds the movableportions of the MEMS light modulators. Suitable working fluids includethose described with respect to fluid 530, without limitation.Additional suitable fluids are described below. In some embodiments, thefluid is a colorless working fluid that wets the surfaces of the displaycomponents (including front and rear surfaces of moving mechanicalelements) and acts to reduce stiction and improve the optical andelectromechanical performance of the display. Steps 4002 to 4008 can becarried out at room temperature. Room temperature may be a temperaturein the range of about 18° C. to about 30° C., e.g., about 20° C., about22° C., about 24° C., about 26° C., or about 28° C. In an alternateembodiment the filling process is carried out at a temperaturesubstantially above room temperature, meaning a temperaturesubstantially above 30° C.

In an alternate embodiment, the assembly method 4000 may be applied toliquid crystal displays or to electrowetting displays. For liquidcrystal displays the first and second substrates may correspond toportions of an array of liquid crystal modulators, e.g., the activematrix substrate and the color filter plate. The fluid filling step atstep 4008 would comprise the filling of the cell with a liquid crystalmaterial. For electrowetting displays the first and second substratescomprise a control matrix substrate and a black matrix substrate,respectively. The fluid filling step at step 4008 would comprise thefilling of the cell with one or both of the fluids 278 or 280, describedabove.

We will turn next to descriptions of FIGS. 15-20 with continuedreference to the assembly method 4000. FIGS. 15-20 show a display cellas it is sealed according to the illustrative embodiments. Forsimplicity of illustration, some features of MEMS display cells, e.g.,the MEMS light modulators, light guide, reflective layer, have beenomitted from FIGS. 15-20. Details of the construction, operation, andalignment of these features will be understood to include withoutlimitation the features already described with respect to FIGS. 1-10.

FIG. 15 shows a MEMS display cell 4100, with the aperture plate and thefirst substrate substantially parallel. The display cell 4100 comprisesa light modulator substrate 4102, a light modulator array (not shown),an aperture plate 4104, and edge seal 4106, and first and second spacers(4108 and 4110) formed on the substrates 4102 and 4104 respectively. Thespacers on the first and second substrates may be aligned before thebonding of the edge seal at step 4106. The shape of display cell 4100,shown in FIG. 15, is that which a display cell has prior to sealing. Thefirst substrate and aperture plate may be aligned on a parallel platebonder at ambient pressure prior to the fluid fill step 4008. Thedistance between the substrates is determined substantially by theheight of the edge seal 4106, and by the spacer materials which areincluded with the edge seal 4106. The spacers on the aperture plate 4104and on the substrate 4102 are not in contact.

In order to seal the fluid within the display cell, the display cell isfirst removed from the fluid fill chamber, at step 4008, at roomtemperature. Continuing with a description of the method 4000, theequipment holding the displays (e.g., a carrier) is cooled to atemperature in the range of about −15° C. to about −20° C. while keepingthe fill hole of each display cell submerged in the working fluid toprevent bubbles from making their way into the display cell gap. In someembodiments, the carrier is cooled to about 0° C., about −5° C., about−10° C., about −15° C., about −25° C., about −30° C., or about −40° C. Acell press is then cooled to about −20° C. (at step 4010), or about 5°C. to about 0° C., or about 0° C. to about −5° C., about −5° C. to about−10° C., about −10° C. to about −15° C., about −20° C. to about −25° C.,about −25° C. to about −30° C., about −30° C. to about −35° C., or about−35° C. to about −40° C. In alternate embodiments the cell press can becooled to any temperature in the range of −about −10° C. to about −25°C. In another embodiment the cell press can be cooled to a temperaturebelow about 0° C. The display is removed from the carrier and placed inthe cell press (at step 4012). The cell press is pressurized (i.e.,external elastic bladders contacting the outside faces of each substrateare inflated) to a pre-determined pressure above ambient pressure suchthat the display cell is compressed (step 4014). As the display iscompressed, fluid is forced out of the fill hole. The fluid is wipedaway and a seal epoxy is dispensed into the fill hole (at step 4016).

While the display cell is still held at the cooled equipmenttemperature, the seal material within the fill hole is at leastpartially cured at step 4020. Such a curing can be accomplished byeither a chemical reaction between the components of the epoxy, or byapplying UV radiation.

FIGS. 16-19 show display cells under compression during step 4014,according to illustrative embodiments. FIG. 16 shows the shape of thesame display cell 4100, from FIG. 15, as it is being compressed by thecell press and as it is being held at the lower temperature. The cellpress compresses the cell such that the majority of spacers 4110 on theaperture plate come into contact with the spacers 4108 on the modulatorsubstrate. The remaining spacers that are not in contact will allow forfurther cell compression, e.g., in response to temperature or pressurechanges, which will further reduce the likelihood of vapor bubbleformation.

FIG. 17 shows the same display cell 4100 in a condition after it hasbeen further compressed at temperatures below the sealing temperature.The display cell 4100 as illustrated in FIG. 17 comprises two differentgaps or separation distances between the substrates 4102 and 4104. Inthe center of the display assembly the magnitude of the gap is indicatedby the marker “B”. The separation “B” is established by contact betweenthe spacers in the interior of the display; the separation “B” issubstantially the same as the sum of the heights of the spacers 4108 and4110. At the edge of the display assembly, a different gap, marked “A”,has been established by the height of the edge seal 4106. The gap “A” issubstantially equal to the height of the edge seal. In embodiments wherethe seal material 4106 is a composite material, the separation “A” isestablished by the height of the spacer materials or beads that areincluded within the edge seal. The height of the gap “A” is preferablyin the range of 8 to 14 microns, although cell gaps in the range of 4 to20 microns can be useful for the purpose. The difference between gaps Aand B may be between about 0.5 and 4 microns, such that “B”<“A”. Becauseof the existence of the two cell gaps “A” and “B”, the substrates inFIGS. 16 and 17 are no longer flat, but instead are bent or flexedsomewhat to accommodate the two different cell gaps.

It is useful to compare the flexures illustrated for display assembly4100 in the FIGS. 16 and 17. The display assembly 4100 in FIG. 17 is ina relatively more compressed position with respect to the same displayassembly in FIG. 16, with a more pronounced flex in the shapes of thesubstrates 4102 and 4104. Relatively more of the spacers 1408 and 1410near the edges of the display have come into contact in FIG. 17. Becauseof the differences in the cell gaps “A” and “B”, the display assembly asa whole has been rendered substantially compressible at lowertemperatures based on the flexure of the substrates. This cellcompressibility is a useful feature for avoiding the formation of vaporbubbles upon changes in environmental temperature.

FIG. 18 shows the same display cell 4100 in which the cell is underfurther compression from the cell pressure or from a colder temperature.Similar to FIG. 17, the display assembly 4100 in FIG. 18 comprises twodifferent cell gaps. The gap height established by the edge seal isgreater than that established by the spacers towards the center of thedisplay assembly. In FIG. 18 both substrates 4102 and 4104 are bent orflexed to accommodate the compression at lower temperatures.

FIG. 19A shows a display assembly 4300, according to an illustrativeembodiment of the invention, in which the spacers are made of an elasticmaterial, i.e., the spacers 4308 and 4310 are formed from materialswhich are chosen for a reduced modulus of elasticity. Such spacers aremore compressive and elastic, thus allowing for additional cellcontraction even after the spacers have come into contact. The spacers'modulus of elasticity may be altered using suitable curing methods. Themodulus of elasticity of standard photoresist is about 7-10 GPa,however, if a suitable curing method is used, the modulus of elasticitymay be reduced further to allow for some compression but still preventthe aperture plate coming into contact with the MEMS shutter.

FIG. 19B shows a version of the same display assembly 4300 underconditions in which the spacers maintain contact with one another evenwhen the cell is allowed to relax at higher temperatures. In someembodiments, the spacers may be compressive and elastic, thus allowingfor cell contraction when the ambient temperature is lowered or the cellis cooled. For these embodiments or at these temperatures, the height ofthe spacers 4308 and 4310 on the aperture plate and/or the firstsubstrate can be substantially equal to the height of the edge seal4306. As described above, the spacers' modulus of elasticity may bealtered usng any suitable curing method. For these embodiments it isuseful when the spacers in the center of the display havecompressibility greater than that of the edge seal, such that at lowertemperatures a difference in gap exists between the center and edge ofthe display—as is shown in FIG. 19A.

The display assembly method 4000 provides for compression of the displayassembly and the sealing of the display assembly at temperaturessubstantially below that of room temperature (at least the steps 4012 to4016.) Substantially below room temperature may be any temperature atleast about 15° C. below room temperature, e.g., below about 0° C. Afterthe cell has been sealed, the process of cell assembly is completed byallowing the assembly to return to room temperature. After the displayis sealed at step 4016, the cell press is first deflated anddepressurized or released (step 4018). The cell retains the shape it wasgiven at the sealing step 4016, because it is sealed and remains under aslight negative pressure. Afterwards, as the cell warms up to roomtemperature the volume of the fluid increases which will press the firstsubstrate and aperture plate apart. During the warm-up phase, thedisplay cell relaxes into its pre-compression shape. The seal materialis at least partially cured for a predetermined time at step 4020. Thepredetermined time is based at least in part on the time needed for theseal epoxy to wick a particular distance into the cell. The display isallowed to warm to room temperature at step 4022, after which time afinal curing step can be applied to the seal material within the fillhole.

FIG. 20 provides another illustration of display assembly 4100 in acondition after the assembly has been released from the press, sealed,and allowed to warm back to room temperature, according to anillustrative embodiment of the invention. The spacers 4108 and 4110 areno longer in contact. The cell gaps at positions A and B aresubstantially the same height. The fluid within the gap has been allowedto expand without large convex distortions in the shape or flatness ofthe substrates 4102 or 4104. This flatter shape improves the uniformityof optical properties in the display. In particular the flatter cellshape improves the off-axis contrast performance for the display.

Each of the above described processes for cold sealing a displayapparatus may advantageously prevent the formation of vapor bubbles atthe expected operating temperatures for the display. For instance, whenthe ambient temperature around the MEMS display cell decreases, thespacers prevent the MEMS substrates (which may be formed from glass)from making contact even as the fluid contracts further. Furthermore, byperforming a cold temperature seal process, vapor bubble formation,which generally occurs at temperatures about 15° C. below the sealtemperature, occurs at an even lower temperature.

Working Fluids

As described above, the space between the first and second substratesforms a (cell) gap, which may be filled with fluid, such as a gas,liquid, or a lubricant. Some examples of a working fluid are describedabove with respect to fluid 530 within display apparatus 500. Furtherdesirable properties of a suitable working fluid are given in Table 2.For example, the fluid should have a low viscosity. Lower viscositiescan be facilitated if the liquid or fluid in the display includesmaterials having molecular weights less than 4000 grams/mole, preferablyless than 400 grams/mole.

TABLE 2 Desirable properties of the working fluid. Parameter or DesiredFluid Property Units Property Motivation Appearance — colorless clearliquid Specific gravity at 25° C. —  <1 Viscosity at 25° C. mm²s⁻¹  <1less viscous, faster actuation Refractive index  ~1.5 match glass orplastic substrate Freeze point ° C. <−40 Allow for lower operatingtemperatures Boiling point at 760 ° C. >100 Less likely to form bubblesmmHg at elevated temperatures/ reduced pressures CTE /° C.  <0.002packaging at colder temperatures Dielectric constant —  >2 Higher giversbetter relative to vacuum performance Dielectric strength V/mil >300Less likely to break down Vapor pressure at 25° C. Torr  <25 Incombination with higher boiling point, likely to lead to less bubbleformation Vapor pressure at 85° C. Torr <500 In combination with higherboiling point, likely to lead to less bubble formation Conductivity S/cm <10⁻⁹

In addition, it is desirable for the fluid to have high purity, onlysmall viscosity changes with temperature, and low reactivity with a sealepoxy, and to be inflammable.

Suitable low viscosity fluids include water, alcohols, fluorinatedsilicone oils, polydimethylsiloxane, hexamethyldisiloxane,octamethyltrisiloxance, octane, diethylbenzene, perfluorocarbons,hydrofluoroethers, or any combination thereof. Suitable low viscositynon-polar oils include, without limitation, paraffins, olefins, ethers,silicone oils, fluorinated silicone oils, or other natural or syntheticsolvents or lubricants. Useful oils can be polydimethylsiloxanes, suchas hexamethyldisiloxane and octamethyltrisiloxane, or alkyl methylsiloxanes such as hexylpentamethyldisiloxane, or any combinationthereof. Useful oils can be alkanes, such as octane or decane. Usefuloils can be nitroalkanes, such as nitromethane. Useful oils can bearomatic compounds, such as toluene or diethylbenzene. Useful oils canbe ketones, such as butanone or methyl isobutyl ketone. Useful oils canbe chlorocarbons, such as chlorobenzene. And useful oils can bechlorofluorocarbons, such as dichlorofluoroethane orchlorotrifluoroethylene. For use in electrowetting displays, the oilscan be mixed with dyes to increase light absorption, either at specificcolors such as cyan, magenta, and yellow, or over a broader spectrum tocreate a black ink.

In some embodiments, it is useful to incorporate mixtures of the aboveoils or other fluids. For instance mixtures of alkanes or mixtures ofpolydimethylsiloxanes can be useful where the mixture includes moleculeswith a range of molecular weights. One can also optimize properties bymixing fluids from different families or fluids with differentproperties. For instance, the surface wetting properties of ahexamethyldisiloxane and be combined with the low viscosity of butanoneto create an improved fluid.

In some embodiments, the working fluid may include, without limitation,alkanes, e.g., octane, heptane, xylenes (i.e., isomers ofdimethylbenzene), ionic liquids, divinyl benzene, toluene (also known asmethylbenzene or phenylmethane)), alcohols, e.g., pentanol, butanol, andketones, e.g., methyl ethyl ketone (MEK), or any combination thereof.

In some embodiments, fluids which include carbon, flourine, and oxygen,may be used as the working fluid. Examples of such fluids includefluorroketone, hydrofluoroether, ethoxy-nonafluorobutane, ethylnonafluorobutyl ether, fluorobutane, fluorohexane, and2-trifluoromethyl-3-ethoxydodecofluorohexane.

In some embodiments, a blend of perfluorocarbons and/orhydrofluoroethers may be used to create an improved fluid.Perofluorocarbons include FLOURINERT Electronic Liquid FC-84, whilehydrofluoroethers include NOVEC 7200 Engineering Fluid or NOVEC 7500Engineering Fluid (all manufactured by and registered marks of 3M).Examples of suitable blends are shown in Table 3. Those skilled in theart will come to realize that other blend compositions may be suitableas the working fluid.

TABLE 3 Illustrative blends of suitable perfluorocarbons andhydrofluoroethers. NOVEC 7200 NOVEC 7500 FC-84 BLEND 1 about 79% about21% — BLEND 2 about 58% about 42% BLEND 3 about 85% about 15% —

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The forgoingembodiments are therefore to be considered in all respects illustrative,rather than limiting of the invention.

What is claimed is: 1-48. (canceled)
 49. A method for manufacturing adisplay assembly including a first transparent substrate and a secondtransparent substrate, comprising: cooling the first and secondtransparent substrate to a first temperature; applying a fluid to atleast one of the first and second transparent substrates, wherein thefluid is applied at an equilibrium vapor pressure substantially the sameas a vapor pressure of the fluid; aligning the first and secondsubstrates; and curing a seal material to seal the fluid between thefirst and second substrates.
 50. The method of claim 49, wherein thefirst and second transparent substrates are cooled to the firsttemperature prior to applying the fluid to at least one of the first andsecond transparent substrates.
 51. The method of claim 50, wherein thefirst and second substrates are held at substantially the firsttemperature while applying the fluid to at least one of the first andsecond transparent substrates, while aligning the first and secondsubstrates, and while curing the seal material.
 52. The method of claim49, wherein the first temperature is below room temperature.
 53. Themethod of claim 52, wherein room temperature is between about 18° C. andabout 30° C.
 54. The method of claim 49, further comprising providing aplurality of spacers connected to the first and second substrates toestablish a gap between the two substrates;
 55. The method of claim 54,wherein the plurality of spacers maintain at least a first gap betweenthe two substrates.
 56. The method of claim 49, further comprisingfabricating an aperture layer on the second transparent substrate. 57.The method of claim 56, wherein applying the fluid includes applying thefluid directly to an open face of the second transparent substrate. 58.The method of claim 57, wherein applying the fluid includes spreadingthe fluid over the open face of the second transparent substrate untilconfined by a perimeter of the seal material.
 59. The method of claim49, wherein the first temperature is below about 0° C.
 60. The method ofclaim 49, wherein the fluid includes a hydrofluoroether liquid or aliquid blend of at least one perfluorocarbon and at least onehydrofluoroether.
 61. The method of claim 49, wherein at least one arrayof MEMS light modulators is formed on at least one of the first andsecond transparent substrate.
 62. The method of claim 49, wherein acontrol matrix is fabricated on at least one of the first and secondtransparent substrate.
 63. The method of claim 49, further comprisingapplying a conductive adhesive to at least one of the first and secondtransparent substrate.
 64. The method of claim 49, further comprisingventing the display assembly to atmospheric pressure.
 65. The method ofclaim 49, wherein the seal material is contiguous.
 66. The method ofclaim 49, further comprising curing the seal material for a first periodof time.
 67. The method of claim 66, wherein the first period of time isbased at least in part on the time needed for the seal material to wicka particular distance into the display assembly.
 68. The method of claim49, further comprising warming the display assembly to substantiallyroom temperature.